Variation of output laser pulse energy has great influence on the measured data of radar, so the output laser pulse energy must be measured accurately in real-time. Aiming at the problem that output laser pulse width is very narrow, and it is difficult to achieve high frequency acquisition circuit, a pulse energy monitoring system is designed based on the peak sampling and holding circuit. In situ monitoring of simulated pulses with pulse width of about 10 ns and repetition rates of 20, 60, 100 Hz is carried out. Results show that the system can accurately detect laser emitting energy without any pulse missing. There is a good linear relationship between the system output voltage and measured laser pulse energy in lidar observation experiments, which provides a correction basis for lidar data inversion.