A computer system and a power management method thereof are provided. The computer system includes a system management controller, a northbridge, a hyper transport bus, a central processing unit and a power management signal line. The northbridge is electrically connected to the system management controller via the hyper transport bus. The central processing unit is electrically connected to the northbridge but does not support the hyper transport bus. The system management controller outputs a power management signal to the central processing unit and the northbridge via the power management signal line. When the system management controller asserts the power management signal, data transmission is stopped with the disconnection of the hyper transport bus, and the central processing unit changes to a power-saving state from a working state.