In laser imaging technology of earth observation, the gray images of terrain objects are produced directly from laser echo information in order to do classification and recognition of the terrain objects. So it is necessary to establish the relative models of laser echo identities and terrain object identities. The power, waveform and other main identities of pulse laser echo from ground are analyzed based on the experimental results of the fundamental system for laser imaging of earth observation. The backward scattered power of terrain objects is discussed. The gray image of part of terrain is produced through laser echo information after airborne information acquiring and data processing. Some elementary relations between laser echo and terrain object are obtained by comparing the gray image and passive infrared imaging result. Some technical supports are provided for 3D laser imaging, classification and recognition of terrain objects.