In linear frequency modulation system, the receiver cannot demodulate the Doppler parameter correctly based on the return signal if the initial phase of the transmitter output is not coherent. A prior frame sequence is introduced to realize the coherence between the initial phase of the output pulse signal and the reference clock by initializing the Direct Digital Synthesis(DDS)’s phase accumulator. The proposed method is simple and practical, because it just only needs to modify the FPGA program without any changes to the hardware circuit. The effectiveness of the method has been verified by the experimental results.