A free space optical(FSO)communication system was designed,which is compatible with 125Mbps polymer optical fiber(POF)LAN system. The FSO system realizes multibuilding communication and extends the transmission distance of POF LAN. The system includes emission and receiving part. The emission part is composed of a 650 nm laser diode(LD),a drive circus and beam alignment and expand lens. The receiving part is composed of a receiving lens and a photo detector(PD). The modulated emission power of LD is 3 dBm and the sensitivity of PD is－27 dBm. The atmosphere attenuation to 650 nm light and the dot whipping caused by air turbulence were analyzed. The signal transmission waveform and communication eye diagram of the system were recorded. The experiment shows that the transmission distance of POF LAN was extended by FSO,which enables more flexibility of the POF LAN installation.