According to the pseudo-random code without repeating rule in the laser guidance signal,introduced a modified method to calculate the least period of the guidance signal based on the traditional continuous minus method.The algorithm found the least period based on the greatest common divisor of the received adjacent threaten pulse time space,and compared with the received signal continually to correct the error,and then deduced the key value which shows when to stop the algorithm.Used the modelsim to simulate the algorithm,and gain the expected decoding and identifying end.In the hardware,implemented based on the XILINX FPGA which indicates the algorithm can identify the least period of the pseudo-random code with receiving a small quantity of guidance signal,and then launches the high frequency spoofing pulse to cheat the enemy accurately.