The 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. This 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 1m， the diameter of the main or the secondary hole should be between 0.3m and 0.6m， and the length of the return-water cavity should be 2~3m 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 15~30m 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℃ and the critical flow rate of injected hot water does not exceed 12L/min.