Deep hot-water drill is an important tool for clean drilling and sampling of subglacial lakes in polar regions. The underground return-water system is the key component of the deep hot-water drill， which mainly contains the return-water hose， heat-injection hose， submersible pump and water cavity. The return-water hose is used to extract molten water from the water cavity to the surface for recycling， while the heat-injection hose is used to inject surface hot water into the water cavity to prevent it from freezing. The thermal and flow characteristics of return-water hose and heat-injection hose are very important for the design of the downhole return-water system， however， it is not be the systematically researched and the variation of pressure loss and temperature loss is still not clear at present. In the paper， the theoretical calculation method of thermal and flow characteristics of return-water hose and heat-injection hose was firstly proposed based on Darcy-Weisbach formula and Sukhov's temperature-drop formula. Then， the numerical simulation method of thermal and flow characteristics of the two hoses was established in COMSOL Multiphysics 5.6 software， and the numerical results matched well with the theoretical calculation results. Finally， this paper systematically analyzed the influence of the factors， such as flow rate， inner diameter， length， water temperature at inlet， thermal conductivity， roughness of inner wall， ice temperature and wall thickness on the pressure and temperature loss of return-water hose and heat-injection hose. The results shows that the pressure and temperature in the return-water hose and heat-injection hose decreases linearly. In common， flow rate， inner diameter and length are the main factors affecting the pressure loss of return-water hose and heat-injection hose， while air temperature， flow rate， inner diameter and length are the main factors affecting the temperature loss. In addition， roughness of inner wall has little influence on the thermal and flow characteristics of the two hoses. When designing the downhole return-water system， the inner diameter of the return-water hose and the heat-injection hose should be increased as much as possible， and the construction depth of the return-water chamber should be reduced. The water temperature at inlet has bigger influence on the thermal and flow characteristics of the heat-injection hose compared with the return-water hose. Generally， the thermal conductivity of 0.4 W·（m·K）^-1 can ensure the return-water hose and the heat-injection hose have good thermal insulation performance and it is no longer necessary to increase the wall thickness to enhance its thermal insulation performance. The temperature loss of the return-water hose is generally not more than 2 ℃， while the temperature loss of the heat-injection hose is about 10~20 ℃. The conclusions above provided an important basis for designing a safe and efficient downhole return-water system.