Spatial modulation (SM) has been employed in indoor visible light communication (VLC) systems because of its high data rate and low complexity transmission. However, the activated LED may have poor channel conditions in conventional SM VLC systems, which leads to significant performance degradation. In this paper, a cooperative incremental hybrid decode-amplify-forward (IHDAF) protocol-based SM-index modulation (SM-IM) VLC system is proposed. For the SM-IM scheme, the spatial information and the modulated information are jointly transmitted in one symbol via the optimal VLC channel. The system model and the theoretical outage probability of the proposed IHDAF protocol-based SM-IM VLC system are presented. Numerical results show that the outage probability of the IHDAF protocol-based SM-IM VLC system can be significantly improved compared with the traditional amplify-and-forward and decode-and-forward protocol-based SM VLC systems. Additionally, the power allocation of the proposed scheme is also studied.