Abstract: To verify dominating processes in the inverse Compton mechanism and the origin of soft photons in the dominating processes, we collected multi-wavelength (quasi-) simultaneity data of 75 flat-spectrum radio quasars from the literature. We fitted the spectral energy distribution using logarithmic parabolic functions, then analyzed three correlations for the whole sample, the subsamples of two bins, respectively. The research results are summarized as follows. (1) We find significant correlations of logU_B(νF_ν)_EC/U_ext(νF_ν)_syn and logδ for the whole sample and subsamples of two bins, respectively. Our results show that the inverse Compton mechanism of the flat-spectrum radio quasar is preferentially dominant by the external Compton process under the leptonic model. (2) Based on the Bootstrap to calculate observed seed factor distribution, we apply this method to constrain the origin of soft photons of the dominant process of inverse Compton mechanism. The observed seed factor distribution suggests that soft photons are likely to be dominated by dust torus for populations with relatively low synchrotron peak frequencies. But, we cannot determine the origin of soft photons of the dominant process of inverse Compton mechanism for populations with relatively high synchrotron peak frequencies.