The invention provides a method, a device and a system for closing down and starting up a computer system. An internal memory of the computer system comprises a phase changing memory (PCM) and a dynamic random access memory (DRAM). The closing down method comprises the following steps of acquiring a closing-down order for closing down the computer system, freezing an active process through an operating system, storing the register data and the cache data, related to the active process, to the DRAM, storing an entire DRAM mirror image to the PCM, and turning off the device power supply so as to close down the computer. The starting up method comprises the following steps of receiving a starting-up order for starting up the computer system, activating the operating system, reading the data directly from the PCM through the operating system, restoring the register data and the cache data, and activating the frozen active process. Compared with the prior art, by utilizing the nonvolatile and high reading efficiency of the PCM, the data can be directly read from the PCM when a computer is started, and the DRAM mirror image no longer needs to be copied from a disk into the DRAM, so that the consumed time can be shortened, and the starting-up speed can be increased.