The invention belongs to the technical field of agricultural equipment and provides a power system of a combine harvester. The power system is characterized by comprising the following mechanisms: a power input mechanism, a gear shift mechanism, a braking mechanism, a steering and clutching drive mechanism and a walking power take off mechanism, wherein axle fit mechanisms which are used for keeping running clearance distances and fixed positions of all the mechanisms are arranged in the power input mechanism, the gear shift mechanism, the steering and clutching drive mechanism and the walking power take off mechanism. The power system of the combine harvester has the advantages that through the axle fit mechanisms, the stability of a crawler transmission structure is guaranteed, failure and damage are not easy, the reduction of maintenance cost is facilitated, the running speed on smooth pavements and adaptability to complicated field parcels of the combine harvester are improved, the cross-region operational performance of the combine harvester is improved greatly, the wear life can be prolonged, the life is further prolonged, and the reliability is further improved.