The intensity modulation direct detection (IM/DD) is widely used for the free space optical (FSO) system currently. In order to explore the most effective and the most practical way to jam the FSO systems, the principle and efficiency of how the low power laser jams the FSO system is researched by using theoretical analysis, simulation and experiment. The feasibility of low-power laser jamming is validated. The jamming phenomenon is mainly manifested as bit error rate increases. At the same time, different principles and phenomena affect by different repetition rates, power and duty cycle of jamming laser on wireless laser communication systems with different communication systems, rates, transmitting power are studied. Because of their different clock systems, synchronous and asynchronous communications have different principles and phenomena. Synchronous communication mainly occurs in the "bit interference" and "clock recovery interference", asynchronous communication system mainly occurs "start bit interference" and "data bit interference." Asynchronous communication that has simple frame structure and coding relatively from synchronous communication can be influenced more easily and serious.