The designed tandem hybrid power box is composed of the input shaft, the intermediate shaft, the output shaft, the shaft gear and a pair of slip gear. In order to reduce the volume of the serial hybrid tractor divider box and meet the larger tillage requirement of the supporting rotary tiller, the mathematical model of the gearbox parameter optimization was established by taking the minimum total center distance and the total volume of the gear box as the objective function under the constraint conditions of the structure, the strength and the stiffness. A parameter optimization strategy (PSO) is adopted based on the improved particle swarm optimization algorithm. The optimization results show that compared with the basic particle swarm optimization algorithm, the improved particle swarm optimization algorithm significantly improves the convergence speed and convergence accuracy, with the improved gear volume reduced by 2.9% and the optimization accuracy increased by 1.5%. The kinematic simulation and test results of the gearbox show that the PTO dynamic performance of the optimized gearbox meets the requirements of the power and the working requirements of the supporting rotary tiller.