To solve the problem of high Peak to Average Power Ratio (PAPR) in Direct Current biased Optical-Orthogonal Frequency Division Multiplexing (DCO-OFDM) system in visible light communication, the selective mapping method can be used to reduce the PAPR of the system. Because the selective mapping algorithm can only select the local optimum in a limited set of phase rotation sequences, the genetic algorithm is introduced to search for the global optimum solution. Meanwhile, the genetic algorithm has the problem of premature convergence. In order to solve this issue, we apply several methods including the adaptive adjustment mechanism, the adaptive multi-point crossover mechanism, the optimal solution protection mechanism and the immediate elimination mechanism to improve the genetic algorithm. Therefore, an Improved Genetic Algorithm- Selected Mapping (IGA-SLM) is proposed. The simulation results show that the proposed algorithm can significantly reduce the PAPR of DCO-OFDM system.