Poly(vinylidene fluoride) (PVDF)-based relaxor ferroelectric polymers, synthesized from the physical or chemical modification of beta-PVDF and P(VDF-TrFE) (TrFE refers trifluoroethylene), show high thickness strain under an external field, which allows them to be employed as electroactive materials for actuation, sensor, and artificial muscle applications. In an effort to disclose the formation mechanism of electrostrain, a series of P(VDF-TrFE-CTFE) (CTFE is chlorotrifluoroethylene) copolymers are synthesized and carefully characterized. By correlating the crystalline and ferroelectric phase transitions to their dielectric, ferroelectric, and electromechanical performances, a new model combining both the Maxwell stress and electrostrictive effect is proposed. The contribution of each effect to the overall strain and its dependence on the electric field, material composition, and the fabrication process of the films have been well addressed. The perfect synergism of two effects is responsible for the large strain under a low field, which offers a strategy to design and fabricate electroactive polymers with excellent electrostrain performances.