According to the problem of the high peak-to-average power ratio(PAPR) in optical orthogonal frequency division multiplexing(OFDM) systems,the principle of PAPR reduction method by clipping is studied.Based on this principle,a clipping noise model of optical OFDM systems in the MQAM modulation is established by analyzing and computing a large number of data.According to the established model,the error-symbol-rates of optical OFDM systems in different modulations can be directly calculated after clipping.The simulation results show that the curve of the established clipping noise model agrees well with that of the simulation model of optical OFDM systems in different modulations.Furthermore,the maximum value of the mean square error(MSE) between the two curves in different clipping ratios is not more than 1.26×10-2,Which demonstrates the feasibility and applicability of the established clipping noise model.