The most important thing in the nonscanning lidar theory is the lidar equation. The classical lidar equation is suitable on the condition that the field of view is very small and the backscattering of the different part of the target is almost same, but it's unsuitable when the field of view is not very small, when the backscattering of the different part of the target is not same but follow the Lambert distribution. This paper analyses the backscattering of the target and gets a modified lidar equation suitable for the condition.