Multi-beam laser is one of the hot topics in aerospace topographic mapping, where the core is the determination of laser pointing. Laser reference cameras (LRCs) are usually used for recording, and the boresight monitoring function and performance of LRCs are the key to ensuring the accuracy of laser pointing determination. Firstly, a method based on a high-stability central prism is proposed to monitor and measure the LRC boresight, and the calculation algorithm is given. Second, an experimental verification system is built, and the correctness of the algorithm is verified with a Hexpod turntable and a photoelectric auto-collimator. The results show that the measurement results obtained by the proposed algorithm are consistent with those obtained by the photoelectric auto-collimator, with the error of less than 0.2″. Furthermore, the thermal drift of the LRC boresight is measured, and it reaches 0.3″ and 1.4″ around the X axis and Y axis, respectively. Finally, the monitoring accuracies of the boresight in different environments are evaluated. The results show that the monitoring accuracy in vacuum environment is 0.02″(one times standard deviation), which meets the practical application requirements of the on-orbit monitoring of the LRC boresight. Compared with the vacuum condition, the error increases 2--4 times under the atmospheric vibration isolation condition and 10--15 times under the non-vibration isolation condition, respectively.