Abstract: Based on the model of External Compton (EC) photons, we have collected a sample of 193 Flat Spectrum Radio Quasars (FSRQs) observed by the Fermi Large Area Telescope and explored the properties of the external photon fields (as radiation seeds) in them.The results of our study are as follows.(1) There is an anti-correlation between (L_IC/L_S) and (ν_IC/ν_S) in our sampled FSRQs.Here, the (L_IC/L_S) is the ratio of the peak luminosity of inverse Compton scattering radiation to that of synchrotron radiation;the (ν_IC/ν_S) is the ratio of the peak frequency of inverse Compton scattering radiation to that of synchrotron radiation.(2) The peak luminosities of inverse Compton scattering radiation are strongly correlated with those of synchrotron radiation in the FSRQs.(3) In most FSRQs the peak luminosities of inverse Compton scattering radiation are above those of synchrotron radiation.The results show that in FSRQs photons as radiation seeds may be from broad-line regions as well as additional regions, and jet motions are related to the magnetic fields/external-photon energy densities.