A method of supplying electric power to a computer system compliant with specification provided by the OCP is provided. The method includes: a PLU setting an OCP_V3_EN signal to a high level so as to make the computer system operate in a first mode; a BMC obtaining card data from an OCP card when the computer system operates in the first mode; the BMC determining whether standby power provided by a PSU in the first mode is sufficient according to the card data; and when it is determined that the standby power is not sufficient, the BMC controlling the PLU to make the computer system operate in a second mode requiring more electric power, and to then control the PSU to provide main power that is greater than the standby power to the OCP card.