Abstract: A compact design of photon counting 3D imaging ladar was proposed, which can be used in full-time remote detection applications but with strict restrictions on the size and weight, such as high-speed vehicle guidance, high-speed UAV, helicopter, ship navigation and obstacle avoidance, and railway track obstacles exploration. This design adopted high sensitivity photon counting technology, common aperture for laser transmission and receives, as well as conjugate optics in the telescope. And the two-dimensional scanning mechanism was placed in the rear of the telescope, to expand the telescope aperture, improve the detecting range and reduce the size of scanning mirror. With a smaller scanning mirror, the scanning speed was further improved. The laser transceiver was designed with a narrow divergence angle and narrow instantaneous common FOV. Combined with the application of ultra narrowband filter, the ladar system can achieve good performance under the strong daylight noise condition. At last, key technologies of high repetition laser emission, emission stray light suppression, polarization degradation, photon counting sunlight background denoising use in this design were discussed.