Layered spatial modulation technique is introduced into free space optical communications. A layered optical spatial modulation (LOSM) system suitable for turbulent channels is proposed by simultaneously activating two lasers using pulse position modulation (PPM) and pulse amplitude modulation (PAM), respectively. The principle of layer mapping and bit mapping is described in detail, based on which the bit error rate expression of the proposed LOSM system is derived, and the Monte Carlo simulation is utilized to verify its reliability. The simulation results show that the LOSM system can greatly improve the spectral efficiency of the system. For example, the spectral efficiency of the (5,4,2,4)-LOSM system is more than 9 times that of the (8,4,16)-SPPM (spatial pulse position modulation) system for the same transmission rate. When the spectral efficiency is 4 bit·s -1·Hz -1, the transmission rate of the (5,4,2,4)-LOSM system is nearly twice that of the (8,4,2)-SPAM (spatial pulse amplitude modulation) system, and the two systems have the same bit error performance.