The inter-satellite laser communication system that implements for acquisition,pointing and tracking (APT) process,is a switch system which influences the continuity of communication widely. The switch sequence and the active subsystem are determined by the spot image collected by charge coupled device(CCD). In this paper,the process of APT is analyzed in detail. Based on the whole workflow,the switch system model is represented. And the influence of spot noise which mixed into spot image to system performance is verified. The suppression algorithm that cascaded by average value wave filtering,Otsu method and selecting connected area method is proposed for the noise elimination. The simulation indicates that the algorithm is effective for pepper salt noise,Gaussian noise and stellar background noise,and the continuous communication can be got.