Aiming at the problems and performance shortage of digital pulse interval modulation (PDM), a novel dual-amplitude fixed length digital pulse interval modulation (DAFDPIM) scheme is proposed. The structure of the modulation is described in detail. The transmit power, bandwidth requirement, transmission capacity and error performance are discussed. Then these performances are compared with several other improved DPIM modulation. The theoretical analysis and simulation results show that this novel DAFDPIM can achieve a good balance on various performances and make up the lack of the DPIM with unfixed symbol length. Hence, this novel modulation scheme is superior in optical wireless communication system.