For the system power supplier, it should be operated under critical mode to maintain its switching stability on the condition of dynamic load. In this paper, the principle of critical mode has been analyzed based on the FLYBACK structure. After the analysis of the pole and zero point corresponding to non-continuous mode and continuous current mode, the design of a critical mode controller, which based on the adjustment of the switching frequency, has been proposed in this paper. The proposal has been simulated using SPICE software. Moreover, the power supply with the function of critical mode controller has been designed in the paper, too.