The invention relates to power parts of farm cultivators, in particular to a gearbox of a farm cultivator. The gearbox comprises a box body; a gear shifting mechanism and a force applying mechanism are arranged inside the box body; and a gear shifting operation device is arranged outside the box body. As the gear shifting mechanism and the force applying mechanism are arranged, power output from general power achieves advancing and reversing through the gear shifting mechanism of a transmission, and variable speed and torque are achieved through the force applying mechanism. According to the technical scheme, compared with the prior art, the gearbox has the following advantages that the structure is reasonable and compact, operation is convenient, gear shifting and force applying are accurate and reliable, and the manufacturing cost is reduced. The problem that speed control of the farm cultivator is simplex can be solved, the use safety of the farm cultivator is improved, and the farming efficiency is improved. When the farm cultivator goes uphill or conducts farming, the traveling speed is reduced by applying force on the gearbox, torsion is increased, the farm cultivator smoothly goes uphill, and the gear shifting function is provided.