The spectral pattern of pulsed laser is one of the most important parameters for atmospheric wind and temperature lidar. Accurate detection of the spectral pattern of pulsed laser can provide scientific and effective data for wind and temperature inversion. Since the repetition rate of the laser is usually a few tens of hertz (the repetition period is a few tens of milliseconds) and the pulse width of laser is only a few nanoseconds, the linewidth measurement of pulsed laser is much more difficult than that of continuous laser. In this paper, a system based on Fabry-Perot interferometer (FPI), narrow pulse signal gate integral averager and modulus sampler is built, which can detect the spectral pattern of pulsed laser. The linewidth of the sodium layer wind and temperature lidar is detected by this system. The output linewidth of the pulsed laser is measured with the system to be approximately 123 MHz, which is basically consistent with the measurement result of 120 MHz in foreign countries. This system can be mounted on the lidar for real-time monitoring of the spectral pattern of laser, which can provide technical support for ensuring the accuracy and stability of the wind measurement lidar.