In this study, an amplitude-modulation phase discrimination scheme was used. A direct digital frequency synthesizer and laser driver were used to generate a frequency-modulated laser with stable frequency. The laser was injected into the optical path to be measured, and then performed photoelectric conversion and amplification after the optical path. With regard to the phase reference signal of the frequency converter, a mixer was used to measure the phase difference between the optical path signal to be measured and the reference local oscillator signal, thereby obtaining the optical path delay information. The main contributions of this study are as follows. First, a differential detection method was proposed for the three-stage phase shift in the local oscillator signal link to optimize the phase detection point and improve the measurement accuracy. Second, a short-term two-phase point measurement mode is used in a single section to effectively reduce the measurement errors caused by the fluctuations in the light source power, light intensity in the optical path, photoelectric detection and amplification circuit gain, and phase difference drift caused by changes in temperature. Third, measurement and sampling were performed multiple times at each phase point; the phase difference was calculate based on the measured average, and the time difference was derived. The functional relationship between the measured voltage and the measured delay and the factors that affect measurement accuracy were analyzed in detail. A verification system was built, and the experimental verification was completed. The experimental results show that the scheme can obtain accuracy of 1 ps within 4-ns time delays, which can greatly improve the synchronous measurement accuracy of the existing high-power laser devices.