The overlap regression of instantaneous field of view (IFOV) arising from dynamic laser scanning sounding was put forth and a concept of dynamic overlap factor,which describes the dynamic sounding effect of scanning laser, was presented. The relation between the dynamic overlap factor and operation parameters, system specifications and its influence on the sounding effect were analyzed in detail. Some special examples were discussed to explain the obtained results. Finally some suggestions were given for designing laser scanning imaging system with better sounding effect in earth observation.