The gearbox is a very important part of the vehicle, it can change the transmission ratio, enlarge the driving torque and speed. With the development of modern technology, gearbox also has upgraded, from the original manual gearbox, to the current stepless gearbox, from no Synchronizer to have Synchronizer, control more and more convenient. At present, widely used in engineering machinery diesel engine, its torque and speed range is small, can not meet the vehicle under various operating conditions for traction and driving speed requirements, the need to use gearbox to resolve this contradiction. The performance of gearbox is the key to measure the power, economy and driving of engineering machinery. The current transmission system mainly includes: mechanical transmission, hydraulic transmission, hydrostatic transmission. The gearbox has the human shift and the power shift, the structure has the fixed axis type and the planetary type.