Abstract: The control mechanism and control strategy of the clutch-to-clutch power downshift process of automotive automatic transmission were studied. Firstly, the power conversion mechanism between the two clutches in the power downshift process was theoretically clarified, and a method was proposed to achieve the ideal power lap between the two clutches, and verified by simulation and real vehicle test. Then the mechanism of generating a shift shock at the clutch fully engaged was revealed, and a strategy was presented for reducing such a shock. Finally, based on the mechanism research, a piecewise control strategy was proposed for the target trajectory of the input shaft speed. The results of the simulation and real vehicle test show the correctness of the mechanism analysis and the feasibility of the control strategy.