Transmission power downshift control method and device of hybrid power vehicle and vehicle

The invention relates to a transmission power downshift control method and device of a hybrid power vehicle and the vehicle. The method comprises the steps that when it is determined that a transmission needs power downshift, the target slip frequency of a separation clutch is determined; the torque of a separation clutch is controlled, so that the difference between the rotating speed of a power assembly and the rotating speed of an input shaft where a target gear of a transmission is located can follow the target slip frequency of the separation clutch, and rotating speed synchronization is conducted; in the rotating speed synchronization process, a transmission is requested to engage a target gear, and after the transmission engages the target gear, a clutch is requested to be connected to charge oil, so that the pressure of the clutch is stabilized at a semi-engagement point; after oil charging of the joint clutch is completed and rotation speed synchronization is completed, the torque of the clutch is sw