A high speed optical communication system based on field-programmable-gates-array (FPGA) is designed, which can be used in ground to satellite communication. FPGA is used as central process, and pulse position modulation (PPM) is selected. A new frame structure is designed to solve frame synchronizing problem, and four separating phase clocks are used to synchronize PPM slots. FPGA′s working clock is decreased and structure of the system is simplified in the new design, and Gray code is used to decrease bit error. The data rates can reach up to 20 Mbit/s, and bit-error-ratio (BER) is 8.9×10-9. The system provides experimental supports for follow-up image data transforming between ground and satellite.