In order to solve the problem of high peak average power ratio (PAPR) in optical OFDM system effectively, a design scheme of optical OFDM baseband system is proposed, which combines optical OFDM baseband system and constant-enveloping OFDM modulation.Constant envelope optical OFDM baseband system modulates the OFDM signal to the phase of the constant envelope carrier signal on the basis of ordinary OFDM modulation, and finally modulates to the optical carrier signal. The PAPR of the optical OFDM signal obtained is reduced to 0dB, which solves the nonlinear problem in optical fiber transmission.In this paper, the mathematical model of constant envelopment OFDM baseband system is built in Matlab for simulation, which is implemented on FPGA hardware, and the optimal modulation index is found in the built system.The experimental results show that the PAPR can be reduced to nearly 0dB in the constant envelopment OFDM base-band system in practice, and the system obtains the highest spectral utilization when the modulation index is less than or equal to 1, and the system obtains the lowest bit error rate when the modulation index is 1.3.