Abstract: For the structure of topological for Flyback switching power supply, a simple and practical valley detection circuit was put forward based on the research of quasi-resonance and soft switching by predecessors,which could accurately detect the voltage valley over a wide resonant frequency range greater and a full load range. Spectre simulation with TSMC_0.5 μm BCD process shows the circuit could accurately detect the voltage valley for different loads and resonant frequency under 2 MHz(relative error less than 1‰).