In high-speed continuous-variable quantum key distribution system, frame synchronous technology is one of the most important technologies to ensure accurate access to effective information carried by quantum signals. A simple and efficient frame synchronization theoretical method based on the measurement results of the quadratures of coherent states with balanced homodyne detector is proposed. This synchronous method in the continuous-variable quantum key distribution system is experimentally realized with repetition frequency of 25 MHz based on Gaussian modulation coherent states, and the performance of the proposed synchronous method is analyzed in detail. The results show that the proposed frame synchronization method can effectively overcome the disturbance from the environmental factor in the key distribution of quantum signals, and thus realizes efficient frame synchronization between the two communicators.