A real-time monitoring system for center frequency of pulse laser is designed and built based on the optical heterodyne method. The single-frequency nanosecond pulse laser to be measured is mixed with a frequency-shifted continuous reference beam whose spectrum characteristics are known, and a light intensity signal is generated, which is then recorded by a photodetector and a data acquisition card. The center frequency information of the pulse laser can be calculated according to the spectrum of the beat signal. Measurement rate and precision of this system are evaluated through the method of frequency shifting, chopping and self-heterodyne, which uses a 1.57 μm continuous single-frequency laser as a reference beam. Experimental results show that the response time of the system is 6 ms, and the root-mean-square error is less than 0.07 MHz when the width of laser pulse is around 30 ns and the sampling rate is 2 GSa·s-1.