This paper presents a signal synchronization algorithm under ulter-low signal-to-noise ratio(SNR).On the one hand,it uses double spreading to ensure the total length of the synchronization segment. On the other hand,it adopts double correlation and differential techniques to counteract dynamic effects,achieving signal synchronization when the SNR is less than -20dB.The algorithm was simulated in MATLAB and finally implemented on the Fudan Microelectronics FM7VX690T device.