To reduce Peak to Average Power Ratio (PAPR) in CO-OFDM systems, bat algorithm was applied for optimizing the sub-carrier phase. Moreover, the algorithm was improved aiming at the problems that the bat algorithm is easy to fall into local optimum and has low convergence precision. In the velocity updating formula, the bat's self study part, the inertia weight and learning factor were introduced to ensure the algorithm convergence speed and improve the convergence precision of the algorithm at the same time as much as possible, avoiding the occurrence of premature convergence. The simulation was carried out in a 100 Gb/s, 4QAM CO-OFDM system, and the simulation results show that the improved algorithm has been obviously improved in searching ability and accuracy. The PAPR can be reduced by about 5.48 dB and 0.52 dB compared with ones of the original OFDM and the BA-OFDM, respectively. The improved algorithm shows better effects on reducing PAPR.