In order to efficiently and accurately measure the phase retardation of wave plate and the azimuth angle of fast axis at the same time, to realize the integration and automation of measuring system, a wave plate measuring system based on the combination of the photo-elastic modulation and the digital phase-locked technology was designed. The detecting laser was modulated by the photo-elastic modulator. By means of digital phase-locked technologies based on FPGA, the first harmonic term and the second harmonic term of the modulated signals were extracted. The phase retardation and the azimuth angle of fast axis of wave plate were demodulated by optimized algorithm. The stepper motor drives the wave plate to rotate to make the fast axis of wave plate reach the zero position. And the phase retardation was displayed by LCD. The experimental system was set up to measure the quarter-wave plate. The experimental results show that the measuring accuracy of azimuth angle of fast axis of quarter-wave plate is better than 0.31° and the measuring accuracy and repeatability of phase retardation of quarter wave plate are better than 99.47% and 0.14°, respectively. The driving signal of the photo-elastic modulator and stepper motor and the processing of the data were controlled by FPGA. The measuring system realized optical, mechanical and electronic integration and the automation of the measuring process.