We present an ARQ-FSO system,which consists of pulse position modulation (PPM) hard decision and a novel automatic repeat request (ARQ) protocol.Instead of traditional ARQ's data error detection,the PPM hard decision results are used to find data error.Then the transmitter only retransmits the failed decoded portion of the packet.These measures can eliminate the information redundancy of traditional ARQ and enhance the system efficiency.Theoretical analysis shows that the ARQ-FSO system can effectively improve the BER performance of the direct hard decision PPM system.Additionally,the proposed system significantly improves the average throughput efficiency compared with traditional ARQ systems.These characteristics make this ARQ-FSO system suitable to apply in low BER and low complexity FSO communications.