For relayed collaborative communications have weak signal of direct paths between the transmitter and the receiver and low Signal-to-Noise Ratio （SNR）， a Reconfigurable Intelligent Surface （RIS） assisted cooperative Index Modulation （IM） system of Decode-and-Forward （DF） relay （RIS-DF-IM） was proposed. In RIS-DF-IM， as smart Access Points （APs）， RISs were adopted as part of the transmitter at the source and relay nodes to perform phase compensation for the reflected channel to maximize the receiving antenna SNR according to the transmission information， and perform IM on multiple antennas of receivers of the relay and destination nodes to improve the spectral efficiency of the system. At the same time， the theoretical union bounds about the Bit Error Rate （BER） of the proposed dual-hop system were solved by using the Moment Generating Function （MGF） method. Besides， a Simplified Pre-greedy Maximum Likelihood （SPML） detector was proposed to reduce the computational complexity by decreasing the number of traversal antenna indexes and simplifying the Maximum Likelihood （ML） decoding criterion formula. Monte Carlo simulation results show that， when the number of RIS elements is 128 and the spatial modulation is adopted， the BER of RIS-DF-IM is about 10 lower than that of the cooperative spatial modulation system where RIS is not connected to the transmitter at the far end； and the BER is dramatically decreased by about 20 compared with the traditional precoded spatial modulation system. Although SPML detector has the BER increased by about 1.4 compared to the Maximum Likelihood （ML） detector， the computational complexity is reduced by a half， achieving an effective balance between BER and complexity.