A computer system on/off test apparatus includes a parameter setting and test control circuit, a test indicating circuit, a voltage converting circuit, and an on/off signal switch circuit. The parameter setting and test control circuit includes a micro control unit (MCU) to set a number of test times and control an on/off test of a motherboard. The test indicating circuit is operable to display the number set by the parameter setting and test control circuit, and display a successful number of test times of the on/off test. The voltage converting circuit is operable to convert a first voltage received from a first standby power connector to a second voltage. The on/off signal switch circuit is operable to switch the first and second voltages according the control by the MCU.