A new fixed-length digital pulse interval modulation (FDPIM) is presented on the basis of pulse position modulation (PPM) which needs symbol synchronization and digital pulse interval modulation (DPIM) with unfixed symbol length. After introducing symbol structure, average transmit power and bandwidth efficiency are studied. Based on weak turbulence channel model, the packet error rate is derived, and compared with on-off keying (OOK), PPM and DPIM. Simulations show that error rate of FDPIM is bigger than PPM and DPIM, but obviously less than OOK. Moreover, FDPIM, which do not need symbol synchronization and has fixed symbol length, can simplify the receiver and solve modulator waiting or buffer overflow in comparison with PPM and DPIM respectively. Hence, FDPIM has some advantages in wireless optical communications system.