In view of the problem of false alarm in imaging space-based laser warning system, the effects of sunlight and lightning on the threaten laser detection and attack event determination are studied by analyzing and calculating the radiant energy density and space-time feature of imaging spot, respectively. The results show that the main false alarm resourses of spacebased laser warning system are sunlight and lightning. The sunlight should exposure the detector directly in one ninth of the satillite orbital period, and the imaging spot of sun is similar to the attack laser. The lightning imaging spot is similar to the illumination laser. About 1.4 lightning events can occur in the field of view (FOV) of the warning system per second. It could not discriminate spots of sun, lightning and threaten laser by the frame subtraction technology.