Based on Mindlin's first-order plate theory, the high-frequency vibrations of AT cut quartz crystal plate covered with unequal thickness electrode layers was analyzed. The dispersion relation and frequency spectrum of vibration modes including thickness-shear vibration mode, flexural vibration mode, face-shear vibration mode, extension mode and thickness-twist modes were obtained. The research results show that due to unequal thickness of the upper and lower electrode layers, the two originally groups of vibration modes that were previously considered uncoupled are now coupled and must be solved jointly. The computing results show that the frequency of every vibration mode decrease due to the mass effect of the unequal thickness electrode layer. According to the frequency spectrum of high frequency vibration of quartz crystal plate, the optimal length/thickness ratio of quartz crystal plate can be selected as the manufacturing size of resonator. The relationship between the thickness of the electrode layer and the frequency variation can be used to guide the design of the electrode layer thickness of the resonator.