Provided is a control circuit of a buck converter, comprising three transistors, seven resistors and a comparator. Also provided is a server. In this solution, when a phase voltage of a buck converter changes, a controller in the buck converter is controlled to output a signal for turning off a lower MOS transistor, so that after the signal is transmitted through the line, the lower MOS transistor can be controlled to be exactly turned off just when the current is reversed. Such an accurate reverse current detection function can reduce the voltage loss of the buck converter, thereby improving the efficiency of a system in standby or having a light load.