For the high peak-to-average power ratio (PAPR) of coherent optical orthogonal frequency division multiplexing (CO-OFDM) system, a novel joint technique which is the combination of iterative partial transmit sequence (IPTS) and clipping technique is proposed. Simulation results demonstrate that the PAPR and bit error rate (BER) performance of the proposed technique both outperform those of the single techniques. Under the same conditions, the threshold value and peak power of IPTS clipping joint technique are optimized by 3.44 dB and 0.86 dBm compared with those of IPTS techinque, respectively. At the BER of 10_-3, the optical signal to noise ratios (OSNRs) of the novel joint technique after 320 km and 400 km single-mode fiber (SMF) transmission are 0.68 dB and 1.18 dB smaller than those of clipping technique, respectively.