The invention discloses a gear box of a heavy CNC vertical turning machine, which comprises a motor base, a gear box body, an intermediate shaft in the box body, an intermediate gear which is arranged on the intermediate shaft and a power input mechanism which is arranged between the intermediate shaft and the motor. The box body on the intermediate shaft section is stretched downwards, the box body on the motor base section moves synchronously, and the two sections of the box body are transversely jointed to form a step-shaped lower box body; and the lower end of the intermediate shaft is lengthened to reach into the lower box body, and the intermediate gear arranged on the intermediate shaft moves downwards to drive the power input mechanism arranged between the intermediate shaft and the motor base to move into the lower box body. The gear box of the heavy CNC vertical turning machine has the advantages of high precision, high transmission efficiency, high transmission rigidity, simple and compact structure, low manufacturing cost, low noise, high reliability and high driving force, can enable a worktable to realize a minimum speed of 0.15r / min, a maximum rotating speed of 125r / min and a torque of 1,000 kN.m, can be provided with a 50-160kW motor and has the capability of driving a ultra-large vertical lathe worktable with a bearable load of 1,000 tons.