In deep-space optical communications, the pulse-position modulation (PPM) is a common way of modulation. But in the conventional multiple pulse-position modulation (MPPM), the number of transmitted light pulses per signal block is fixed. With the development of deep space exploration technology, the requirement of the modulation mode must be increasing. It is necessary to improve the MPPM to meet the requirements of deep space optical communication. So we propose a new modulation scheme (n pulse XPPM) to improve both the symbol-error rate (SER) and the bandwidth-utilization efficiency. Based on the transmission characteristics of XPPM, the modulation rate, required power and bandwidth are derived and compared with L-level pulse-position modulation (LPPM) and MPPM. At the same average power, the XPPM scheme achieves much lower levels of SER than the ordinary MPPM scheme. And n pulse XPPM has more advantages than the two other PPM ways in terms of the transmission rate and the required bandwidth. Results of the simulated experiment reveal that the transmission rate and the bandwidth-utilization efficiency of system can reach 2.746 Mbit/s and 85% or more, respectively, when the communication distance of deep space optical communication system from the earth to the mars is 4×1011 m using XPPM.