We developed a novel photorefractive (PR) polymer PVK-PBA:DR1:TNF, andits film sample was prepared with outstanding performance by using themethod of combination of vacuum saturated vapour resolving with vacuumhot-pressing. The sample exhibits distinctive PR properties with thetwo-beam coupling coefficient up to 140 cm−1 and four-wave mixing(FWM) diffraction efficiency above 1.5% in the absence of appliedexternal electric field. The designed experiments, includingmeasurements of the second-order nonlinear coefficient andbirefringence as well as the relationship between the diffractionefficiencies of the FWM and the external bias field, were performed tounderstand the underlying mechanism, because this phenomenon cannot beexplained by the conventional PR theories. It is presented that becauseof its high orientational mobility and large dipole moment, the polymerproduces the photovoltaic effect under the irradiation of laser toinduce a space-charge field so as to engender the PR effect under zero orlow external fields. A model based on the assumption was establishedand the simulation agrees well with the experimental results.