The utility model discloses a transmission shaft twist-off prevention device which comprises a speed reducer and a rotary screen, the speed reducer comprises an output end and an output main shaft, the rotary screen comprises an input end and an input main shaft, the output end and the input end are connected through a transmission mechanism, and the transmission mechanism comprises a connecting cylinder, a first gear disc, a second gear disc, a first chain and a second chain. The connecting cylinder sleeves the output main shaft and the input main shaft, the first gear disc is fixedly connected to the end part of the output main shaft, and the second gear disc is fixedly connected to the end part of the input main shaft; the first chain and the second chain are meshed with the first geardisc and the second gear disc respectively, arranged side by side and fixedly connected. The problems that a speed reducer is difficult to disassemble, a shaft is bitten, and an equipment shaft is broken are solved, and construction cost and labor intensity can be saved.