For oil shale in-situ mining, mine shaft construction and foundation pit support, underground frozen wall should be built to prevent underground water flowing into the mining area and the leakage of oil and gas. Cooling liquid temperature of underground frozen wall not only affects freezing closure time, but also affects the refrigeration efficiency of the underground frozen wall and further affects the entire costs of the construction and operation. By the finite element simulation and the theoretical analysis on the underground temperature field in the refrigeration process, the influence law of cooling fluid temperature on the underground temperature field was found and the cooling liquid temperature was optimized. It was concluded that under refrigeration modes of refrigeration unit or air cooling, the appropriate temperature of cooling fluid should be -10 to -15℃, which could be the theoretical guidance to improve the refrigeration efficiency of underground frozen wall.