In this paper, an online energy monitoring system for laser rangefinders is designed to solve the problem of the difficulty of direct manual measurement of laser rangefinder emission energy under unattended conditions. In order to prevent the laser energy decline affects the performance of the laser rangefinder, it is necessary to monitor the working status of the laser rangefinder in real time. The structure of the energy probe and detection position was studied, and the hardware circuit was designed to measure the emission laser energy of rangefinder. The online energy monitoring system designed in this paper can realize the wide-range real-time energy monitoring of kHz repetition frequency laser rangefinder without affecting the performance, and achieve miniaturized and integrated requirements.