A computer system with power management and the method thereof. First, the CPU outputs a power management signal to the south bridge. The south bridge responds with a stop clock signal, and then the CPU responds with a stop grant message. The north bridge receives and analyzes the stop grant message to identify a power supply mode. If the power supply mode is to suspend the main power supplied from the power supply, the north bridge outputs a state transition signal to the peripheral, which then responds with an acknowledge signal. The north bridge passes the stop grant message to the south bridge after receiving the acknowledge signal. The south bridge receives the stop grant message and outputs a power control signal accordingly. The power supply receives the power control signal for suspending the corresponding power accordingly.