The invention discloses a farm machinery integrated four-gear transmission method. A farm machinery integrated four-gear speed changing box is adopted to implement gear shifting transmission, power is transmitted to a speed changer input shaft of the farm machinery integrated four-gear speed changing box through a clutch, by means of engagement of a gear shifting drive gear and a gear shifting driven gear and by means of combination between a gear shifting splined hub and a combined sleeve, four-gear transmission is achieved, a gear shifting structure and a planet main speed reduction device are integrated, the integration degree of the speed changing box is high, the weight of the speed changing box is effectively reduced, meanwhile, related accessories are also reduced, assembling and dissembling are easy and convenient, the gear shifting operation and switching are flexible, the transmission efficiency is high, the power loss is low, the independent matching performance is good, and the farm machinery integrated four-gear transmission method can be compatible with various machine types to meet needs of modern diversified farm equipment.