A mechanical interlock transmission device for dual-power automatic transfer switches is provided, which comprises a motor, a speed reducing mechanism, upper and lower fixed plates, a transmission main shaft, first and second slippers, first, second, third and fourth micro-switches, and a cam rotary disk. The invention is characterized in that the device further comprises a solenoid valve, a main gear, a main gear shaft, and an intermediate shaft, wherein the solenoid valve is fixed below the upper fixed plate and is provided inside with a solenoid valve plug and a second return spring; the main gear is fixed below the upper fixed plate by the main gear shaft and engaged with a pinion of the speed reducing mechanism; the top end of the intermediate shaft is inserted into a gear shaft hole of the main gear shaft and the bottom end thereof is covered with a first return spring and then inserted into a central shaft hole of the transmission main shaft; the middle part of the intermediate shaft is provided additionally with a clutching toothed block seat on which a clutching toothed block is formed and engaged with a clutching toothed socket of the top of the transmission main shaft. The invention has the advantages of small motor rotation angle, short switching time, improved operational reliability of the circuit breaker, long service life, and easy manual operation.