In order to solve the problem of symbol synchronization in pulse position modulation (PPM) and modulator waiting or bugger overflow resulting from the unfixed length in pulse interval modulation (DPIM) in optical wireless communications, a novel fixed length dual-amplitude pulse interval modulation(FDAPIM) is discussed. The symbol structure, transmission power, bandwidth requirement and error performance of FDAPIM in a non-dispersive channel are presented and compared with OOK, PPM, DPIM and FDPIM. Simulations show that the packet error rate of FDAPIM is higher than that of PPM and DPIM, which is similar to FDPIM and less than OOK obviously. Moreover, FDAPIM, which does not need symbol synchronization and has fixed symbol length, helps to simplify the receiver and solve the problem of modulator waiting and bugger overflow. It is superior in wireless optical communications system.