A free space optical communication system has been developed. In this system ,two transmitters and two receivers are employed. In each of the two transmitters ,a compound collimation optical system has been used , and the source is an 980nm laser ,which is modulated by its current directly. In each of the receivers ,we use a big aperture and long focal length lens and 2 reflecting mirrors. Two receivers share a half-reflected-half-transparent mirror ,a PIN detector for signal detection and a four-quadrant detector for aiming at each other.With this system ,622Mbit/ s air communication has been realized between point to point of a distance 100m to 2km in normal weather condition in Beijing. This paper presents details of optical systems in transmitters and receivers.