In permafrost and ice engineering， the study on mechanical properties of ice is of great importance， wherein changes in content of unfrozen water inside ice can lead to overall changes of properties of ice. At present， the study at microscopic molecular scale on control factors of unfrozen water content in ice presents non-adequate. As a relatively special solid material， ice can be classified into single-crystal and polycrystalline ones in terms of their morphological difference， the latter of which usually exists in a polycrystalline structural state， in nature and artificial laboratory environments. And the structural difference between polycrystals and single crystals is mainly that the former have grain boundaries， that is， some relatively disordered or less neat regions in the middle of two regularly arranged lattices. In contrast， the entire structure of a single crystal is a long-range ordered crystal structure. Namely， polycrystals can be viewed as being composed of many single crystal particles. The exploration on mechanical properties of polycrystalline ice basing single-crystal ice can help us to further understand its microstructural changes. In this study， after building single-crystal and polycrystalline ice models， setting up molecular force fields， and initializing the system for models， uniaxial tensile and compressive tests were implemented on single-crystal and polycrystalline ices by molecular dynamics simulations， under five temperatures， three strain rates， four grain sizes and three heating rates. The mechanical properties of ice crystals and internal microstructural changes inside them were then investigated， under different influencing factors， aiming to reveal the underlying influencing mechanism of unfrozen water proportion on mechanical properties of ice crystals. In terms of simulation results， for single-crystal ice， mechanical properties of them appear weak relation to unfrozen water proportion， but significant dependence on breakage degree of six-membered ring structures， as well as they are influenced by temperature， strain and strain rate. And during both tension and compression tests， single crystal ice shows significant brittle damage， i.e. the strength increasing along with temperature decreasing or strain rate increasing. In addition， the compressive strength of it tends to be higher than tensile one， but shows an obvious nonlinear mechanical response after the yield point. By contrast， due to the existence of grain boundaries， mechanical properties of polycrystalline ice are more sensitive to the unfrozen water ratio and the ice crystal strength increases along with decrease of unfrozen water ratio， which is mainly controlled by temperature， grain size and its interfacial state. The smaller the crystal size is and the larger the grain boundary area is， the easier the unfrozen water forms at the grain boundary. And the mechanical properties of polycrystalline ice change drastically under the influence of unfrozen water ratio. Moreover， due to strain-induced amorphization and collective sliding between grain boundaries， polycrystalline ice with nanograins is unstable and the sensitivity of it to temperature and strain rate is more pronounced in comparison of that of single-crystal ice， indicating structural changes between grain boundaries play a non-negligible role in mechanical properties of polycrystalline ice. In addition to elastic deformation， the combination of grain boundary slippage， grain rotation， amorphization and recrystallization dominate the plastic deformation of polycrystalline ice. Under influence of external forces and warming， the crystal structure in polycrystalline ice crystals gradually changes， from stable hexagonal ring to unstable quadruple ring， five element ring， seven element ring and disordered water molecule structures， etc. Along with the increase of proportion of unfrozen water， the ultimate strength of ice crystals decreases. In this study， relying on molecular dynamics simulation， the authors focus on generation and changes of unfrozen water at the molecular scale during process of ice crystal deformation and also the intrinsic mechanism of influencing effect of unfrozen water on mechanical properties of ice crystals， with an ultimate hope of explaining the acting mechanism behind some macroscopic phenomena， from the perspective of molecular structure changes.