A family of poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorodifluoroethylene) [P(VDF-TrFE-CDFE)] terpolymers was synthesized. The effect of the chlorodifluoroethylene (CDFE) concentration on the microstructures and ferroelectric properties of the terpolymer has been systematically investigated. It is found that P(VDF-TrFE-CDFE) can be gradually converted from a normal ferroelectric into a ferroelectric relaxor with the increase of the CDFE content from 0.7 to 4.5 mol %. Fouriertransform infrared spectroscopy and wide-angle X-ray diffraction reveal that CDFE is incorporated into the crystalline region and breaks up large ferroelectric domains into nanoregions by stabilizing the gauche conformation. As a result, CDFE eliminates the energy barrier of the ferroelectric-paraelectric transition and leads to the relaxor behavior of the terpolymers at high contents of CDFE, including diffuse dielectric peaks, high dielectric constants (e.g. K = 30@room temperature), and slim polarization hysteresis loops. This work identifies a new structure of the ferroelectric relaxor and provides insights into the role of chlorinated comonomers in the polyvinylidene fluoride-based ferroelectric polymers.