Abstract: Coherent optical orthogonal frequency division multiplexing (CO-OFDM) system is sensitive to nonlinearity because of high peak-to-average power ratio (PAPR) and closely-packed subcarriers, which seriously suppresses the transmission performance.A method for reducing the PAPR was put forward based on constellation restoration, the system model and action mechanism were analyzed. Analyses and simulation results show that, the system Q value of constellation restoration based CO-OFDM system is improved by about 3 dB for 720 km distance at a speed of 10 Gbit/s transmission through standardsingle-mode fiber (SSMF),compared with the CO-OFDM system without nonlinearity suppression. Meanwhile, the clipping factor is strongly influenced by the chromatic dispersion which can decrease the effect of constellation restoration. At the length of 240 km, the clipping factor is 0.9 and 0.8, while the dispersion factor is 12 ps/nmkm and 6 ps/nmkm,respectively.