Abstract: Digital phase loop amplifier is based on phase sensitive detection technology, which can accurately measure the amplitude and phase information of periodic signal. Weak signal often contains noise factors such as DC component, linear component and noise component, which will have a great impact on the amplitude measurement of periodic signal in weak signal. The possible error of weak signal is analyzed, and the PLL algorithm is optimized. The feasibility of the algorithm is proved by MATLAB simulation analysis. Finally, by simulating current signal and adding noise signal into it, and amplitude is measured and analyzed by digital phase locked loop amplifier. The error is about 0. 5%.