On the basis of the advantages of photo-elastic modulation, a phase-modulated ellipsometry combined a photo-elastic modulator with an electro-optic modulator is proposed. It uses one single detection light path to obtain the signals.By switching two work conditions of the electro-optic modulator, it takes lock-in signal processing technology to solve out the two ellipsometric parameters, amplitude ratio Ψ and phase difference Δ, from the p component and s component of reflection light of a simple. The principle of the new type of ellipsometry is analyzed, and a principle prototype is set up. The relationship between the peak value of modulated voltage and the phase modulation amplitude of the photo-elastic modulator is calibrated, and the calibration result is better than 99.95%. Then, an instrument offset method is used to calibrate the experimental system. Finally, a quartz glass reflective sample is analyzed by using this system, and the measurement accuracy of the Ψ and Δ are respectively better than 0.08° and 0.81°. The experimental results show that the instrument offset method effectively eliminates the detection errors introduced by the residual birefringence of the photo-elastic modulator and electro-optic modulator, and the data acquisition time and signals processing time are in the order of the millisecond. In conclusion, this phase-modulated ellipsometry proposed has potential advantages of wide spectrum measurement, stable operation, high repetition rate, fast measurement speed, low cost, and is conducive to industrial automation integration.