The signal process of a laser seeker was analysed on time domain when it was interfered by a laser cheating jammer. A laser guidance System with countermeasure acting as an interfered target and a laser cheating jammer acting as an interfering source,the interactional effect between them was tested from the view of systems on field. The angle errors between the sight of target and the optical axial of a laser seeker were used to characterize the interactional effect. The results show that the effect of a laser cheating jammer on a laser seeker closely relates to the output power of the jammer and the performance of the cheating signal and environmental elements and the methods of signal processing of a laser seeker.