  The external Compton (EC) model is used to study the high energy emission of
some blazars, in which the external photon field is considered to dominate
inverse Compton radiation. We explore the properties of external photon field
through analyzing the FERMI LAT bright AGN sample within three months
detection. In the sample, assuming the high energy radiation of low synchrotron
peaked blazars from the EC process, we find that the external photon parameter
Uext/\nuext may not be a constant. Calculating synchrotron and inverse Compton
(IC) luminosity from the quasi-simultaneous broadband spectrum energy
distributions (SEDs), we find that they have an approximately linear relation.
This indicates that the ratio of external photon and magnetic energy density is
a constant in the comoving frame, implying that the Lorentz factor of the
emitting blob depends on external photon field and magnetic field. The result
gives a strong constraint on the jet dynamic model.
