In order to evaluate the performances of the intersatellite optical communication (IOC), the beam vibration, transmitting loss and thermal noise should be analyzed In this paper, with the uniform analysis of beam spreading, random vibration, avalanche noise and thermal noise, the formula of sensitivity is given. And the laser transmitting factor (LTF) is introduced to describe the beam spreading and vibration· Utilizing the result, the effects of Gaussian beam on IOC are analyzed, and the means of the decision of system parameters are presented.