A signal synchronization algorithm under ultra-low Signal-to-Noise Ratio(SNR) is presented. 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 -20 dB. The algorithm was simulated with Matlab and signal synchronization algorithm under ultra-low Signal-to-Noise Ratio (SNR) is ultimately implemented on the Field-Programmable Gate Array (FPGA) device.