The invention discloses a motor main shaft. The motor main shaft comprises a waterproof ring, a front cover, an angular contact bearing, a front bearing pedestal, a sleeve, a disc spring, a pull rod,a motor stator, a motor rotor, a core shaft, an encoder mounting plate, an encoder, an oil cylinder sleeve, a piston, an oil cylinder cover, a cutter block, an encoding wheel, a rear cover and a rearbearing pedestal; the motor rotor is integrated with the core shaft, and the motor stator is fixedly connected with the sleeve; the front bearing pedestal and the rear cover are arranged at the two ends of the sleeve; the sleeve is in rotation connection with the core shaft through the front bearing pedestal, the rear bearing pedestal and the angular contact bearing; the front cover is arranged atthe front end of the front bearing pedestal; the oil cylinder sleeve is arranged at the rear end of the rear cover; the piston is arranged in the oil cylinder sleeve; the oil cylinder cover is arranged at the rear end of the oil cylinder sleeve; the piston is located at the rear end of the pull rod, and the cutter block is arranged at the tail end of the pull rod; the encoder mounting plate is installed on the rear cover; the encoder is installed on the encoder mounting plate; the encoding wheel is installed at the tail end of the core shaft; and the pull rod and the disc spring are arrangedin a central hole of the core shaft. According to the motor main shaft, the rotating speed, stability and rigidity of the main shaft are improved, and the space is saved.