The invention discloses a liquid CO2 real-time preparation system for underground fire prevention/cooling and a use method, and belongs to the technical field of coal mine cooling and fire prevention.The liquid CO2 real-time preparation system comprises an exhaust gas purification and separation unit of a pithead power plant, an evaporator, a refrigerant circulation system, a wind flow heat exchanger, a local fan, a compressor, a gas-liquid separator, an exhaust gas output unit of the pithead power plant and a process pipeline. A liquid refrigerant outlet of the refrigerant circulation systemis connected with a refrigerant inlet of the evaporator, and a gaseous refrigerant outlet of the evaporator is connected with a refrigerant inlet of the refrigerant circulation system. A CO2 inlet ofthe evaporator is connected with an outlet of the compressor through a gaseous CO2 delivery pipe, and an inlet of the compressor is connected with an outlet of the exhaust gas purification and separation unit. According to the liquid CO2 real-time preparation system for underground fire prevention/cooling and the use method, liquid CO2 can be prepared in the vicinity of an underground goaf, and the long-term continuous implementation of cooling of a working surface and the fire extinguishing operation in the goaf based on the principle of liquid CO2 phase change heat absorption are supported.