On the basis of the modeling of OFDM-ROF system

we obtain the expressions for such performance indexes as Spurious-Free Dynamic Range（SFDR） under external intensity modulation

analyze the effects of the average RF signal power on the OFDM-ROF system and compare it with the single-carrier ROF system.Results show that the average RF signal power should be reduced as much as possible in system design so as to achieve better performances.Under the same RF signal power

the SFDR of the OFDM-ROF system is about 5 dB lower than that of the single-carrier ROF system

so we should take measures in system design to reduce the Peak-to-Average Power Ratio（PAPR） of the OFDM-ROF system.