Abstract: To improve the suppression capability of the background stray light and the small-target detection capability of a laser rangefinder, a field diaphragm matching with the field was added in the receiving optical system of the laser rangefinder. Good results were obtained by the theoretical analysis and the experimental verification. The results show that the field diaphragm has better effect on the stray light suppression, the noise reduction and the detection sensitivity improvement. The ability of laser rangefinder with the field diaphragm was improved to about 1.18 times than that without the field diaphragm. The new laser rangefinder has better ranging capability than before.