A test system and a test method for a Y-branch lithium niobate waveguide phase modulator. The light source is split by a 1×N single-mode fiber coupler to couple the light to the input end of the Y waveguide, and the two output ends are connected to an optical power meter or an extinction ratio tester. The output of the optical power meter or the extinction ratio tester is written into the computer through the data transmission system, and the P~V quadratic nonlinear relationship is fitted according to the test data, and the output voltage V of the optical power meter is converted into the calibrated optical power P, which is substituted into The formula calculates the insertion loss fluctuation and the splitting ratio of the Y waveguide. Add the periodic signal of the required specific frequency, amplitude and waveform to the modulation electrode of the Y waveguide, filter in the modulation signal band, and calculate the residual intensity modulation of the Y waveguide. The invention can continuously sample data, continuously monitor multiple Y waveguide devices, measure parameters such as optical power, insertion loss fluctuation, splitting ratio, polarization crosstalk, and residual intensity modulation, and ensure data accuracy during the sampling process.