Laser altimeter's field of view (FOV) is an important parameter affecting system's performance. The effect of the field of view on the detection performance of system was discussed. Because of the straylight and the sensor's nonuniformity, the FOV changes as the echo's power changes. At the same time the echo's power will be varied with the detecting distance. The testing way of detecting FOV was put forward, and the factual working status was simulated. For system with inner short-range delay protecting circuit, a new method was brought forward. A laser altimeter with the FOV of 1.5mrad was tested. The FOV of the system is 1.46mrad when its detecting distance is 200km and the echo's power is 5×10 -8 W.