Aiming at the disadvantages of peak-to-average ratio and high computational complexity in optical orthogonal frequency division multiplexing (O-OFDM) systems, using the idea of selected mapping (SLM) algorithm and combining the advantages of the discrete cosine transform (DCT) and the inverse fast Fourier transform linearity, an improved SLM peak-to-average ratio suppression scheme— the discrete cosine transform linear selected mapping (DCT-ISLM) method is proposed.  The simulation results shows that compared with the conventional SLM method, the proposed DCT-ISLM method reduces the peak-to-average power ratio by about 3.5 dB. With the increase of the number of candidates signals, the complexity of the system has also been effectively improved. At the same time, the scheme is applied to the traditional O-OFDM visible light communication system, and the error performance is also better than the traditional SLM scheme, thereby further enhancing the practical value of this scheme.