A new modulation scheme called Dual-Duration-Multi-Pulse Position Modulation (DDMPPM) is proposed for free-space optical communication systems by resolving the problems which exist in Pulse Position Modulation (PPM) and Differential Pulse Position Modulation (DPPM). After introducing symbol structure, information transmission rate, bandwidth requirement and average transmission power are studied. Meanwhile, the packet error rate is analyzed based on weak turbulence channel mode. After compared with On-off Keying (OOK), PPM, MPPM, PWM and DPPM, the theoretical and simulation results show that the Packet error rate of DDMPPM, which corresponds to MPPM, is higher than that of PPM and DPPM, but obviously less than OOK and PWM. DDMPPM has no problem such as overflow and underflow in the buffer by fixing the symbol length, and has built-in symbol synchronization. Hence, DDMPPM is superior in free space optics communication systems.