The dynamics of the freezing process in relaxor ferroelectrics are investigated by Monte Carlo computer simulation. The concepts of ``frozen dipole'' and ``slow dipole'' are proposed according to the flipping frequency during the observation time of simulation. The deviation from Curie-Weiss behavior and the frequency dispersion in relaxor ferroelectrics have been proved to be caused by the increasing proportion of slow dipoles. The simulation shows that the external field reduces the dielectric constant and increases the temperature of dielectric maximum, which agrees with the experiment results. The polarization behavior with various electrical and thermal histories and starting from different initial states is also simulated and a systematic explanation based on the bifurcation tree of components in phase space is given. It is proposed that the relaxor characteristics are caused by a gradual freezing process of polar microregions, which arises from the inhomogeneous crystal structure of this kind of material.