Hot-water drill is considered to be the most efficient, safest and cleanest drilling equipment for exploring subglacial lakes in polar regions. A return-water cavity must be built when using hot-water drill to explore subglacial lakes. However, at present, the structure and thermal characteristics of the return-water cavity are still not clear. the paper first sorts out the main structure of return-water cavities of deep hot-water drill. Subsequently, a method for calculating the construction depth of the return-water cavity is established based on the pressure of the overlying ice on the subglacial lakes, and the initial shape of the return-water cavity is determined and the calculation methods for its dimensions is proposed. Then, the methods for calculating the critical temperature of return-water and the critical flow rate of injected hot water are proposed by establishing the physical and mathematical model of the ice temperature field surrounding the return-water cavity. Later, the influence of various factors on the two parameters are systematically analyzed. The research shows that the double-layer main/secondary hole structure is preferred for the return-water cavity when it is used for drilling subglacial lakes. The distance between the main hole and the secondary hole should be less than 1 m, the diameter of the main or the secondary hole should be between 0.3 and 0.6 m, and the length of the return-water cavity should be 2~ 3 m longer than that of the submersible pump. The construction depth of the return-water cavity mainly depends on the thickness of the overlying ice sheet. In practical engineering, the construction depth of the return-water cavity should be 15~30 m greater than the theoretical value. The critical temperature of return-water and the critical flow rate of injected hot water decrease with time. In normal conditions, the critical temperature of return-water does not exceed 2~ 3 °C and the critical flow rate of injected hot water does not exceed 12 L/min.