In order to effectively solve the problem of suboptimal spectrum efficiency of a differential optical spatial modulation scheme (DOSM), a high-dimensional DOSM (HD-DOSM) including spatial, time, and digital signal domains is proposed by using pulse position modulation (PPM) with high energy efficiency and pulse amplitude modulation (PAM) with high spectrum efficiency to construct the space-time dispersion matrix which satisfies the differential process. After the differential coding principle is introduced in detail, the upper bound of the theoretical bit error rate of HD-DOSM is deduced, and its performance is compared with the DOSM system. The results show that the proposed scheme achieves a compromise between spectrum efficiency and error performance. When the spectrum efficiency is 1 bit·s ^-1·Hz ^-1 and the bit error rate is 1×10 ^-3, the signal-to-noise ratio (SNR) of HD-DOSM is improved by about 2 dB, compared with DOSM based on PAM. Meanwhile, the transmission rate of HD-DOSM improves by 2 bpcu (bit per channel use). Based on the sparsity of the transmitted signals, a stepwise detection algorithm is proposed to detect PPM symbols first, which effectively reduces the computational complexity of the decoding algorithm at the receiver. When the PPM order is 4, the complexity of the proposed scheme is reduced by about 75% compared with the maximum likelihood detection algorithm.