Based on the theory of random fluctuation of polarization clusters and the model of Wiener random process, the relaxation law of long-range fluctuation of polarization and the possible forms of light intensity autocorrelation function g2(τ) measured from photon correlation spectroscopy (PCS) experiments have been derived. The relationship between relaxation mechanisms of microscopic polarization clusters and macro relaxation laws is expounded. This research supplies a theoretical model for the application of PCS in researching the relaxation process of polarization clusters in ferroelectrics. Based on the improved He-Ne laser PCS experimental set-up, the relaxation process of long-range fluctuation of polarization clusters in BaTiO3 and 0.71Pb (Mg1/3Nb2/3) O3-0.29PbTiO3 single crystals near phase transition temperature is studied. As for BaTiO3, the dual relaxation processes of long-range fluctuation of polarization clusters are observed at temperatures above TC+4 K, which may be related to its order-disorder mechanism of phase transition. For 0.71Pb (Mg1/3Nb2/3) O3-0.29PbTiO3, the dual relaxation processes exist on both sides of the cubic-tetragonal phase transition temperature. The PCS experimental results are fitted well by the derived theoretical model, and the characteristic relaxation times of long-range fluctuation of polarization clusters are extracted. Two relaxation times, τs and τl corresponding to short and long relaxation time, respectively, are initially observed, where τs is several microseconds, and τl is tens of microseconds. The abrupt increase of relaxation times at phase transition temperature and the phenomenon of critical slowing down can be observed in the two samples.