Real-time monitoring of wavelength is important for high-speed wavelength phase-shifting interferometry. In this paper, a wavelength sensor based on a polarization-maintaining fiber interferometer with four-quadrant demodulation was proposed. We built the wavelength sensing system with resolution better than 0.005 pm and 0.1 ms sampling interval and measured the response time of the tuned wavelength at 35 ms in the phase-shifting process of a commercial wavelength phase-shifting free-space interferometer, as well as the wavelength drift velocity of 0.01 pm per second in the hysteresis process. The optical fiber wavelength sensor with four-quadrant demodulation provides a real-time wavelength sensing scheme for high-speed wavelength phase-shifting interferometers.