The　Forward　Switch　mode　power　supply　is　good　selection　for　power　supplies　of　Unmanned　Aerial　Vehicle　(UAV).　Compared　with　the　conventional　forward　converter,　there　is　one　auxiliary　switch　in　the　active　clamp　forward　converter　to　recycle　the　energy　stored　in　the　transformer　leakage　in　order　to　minimize　the　voltage　stress　of　the　main　switch.　The　power　system　is　designed　and　the　operation　characteristics　of　the　active　switching　power　supply　are　revealed　by　analysis　a　typical　second-order　system　in　this　paper.　Active　Clamp　Forward　Converter　reduce　the　generation　of　EMI,　the　EMC　of　UAV　equipment　is　greatly　improved.　The　results　of　the　analysis　are　verified　by　SPICE　simulation.