It introduces a control circuit of AC asynchronous motor based on electric relay．The circuit controls the strong current through the weak current to realize the start，stop，forward and reverse of single-phase and three-phase AC asynchronous motors．Based on these designs，combined with MCU，sensors and other devices， this research realizes the functions of intelligent irrigation，daylighting，ventilation and temperature control in greenhouse planting and poultry breeding of smart farm，and has a broad application prospect．