Cu2ZnSnS4 (CZTS) has been a hopeful semiconductor material for thin film solar cells due to its earth-abundant and non-toxic elements. However, the photovoltaic performance of CZTS thin film is still inferior by reason of the high recombination rate of photogenerated carriers. Polyoxometalates (POMs), as an electron acceptor, can efficiently capture the photogenerated electrons from conduction band of semiconductors and thus retard the electron-hole recombination. In this work, we fabricate a CZTS solar cell with inverted structure by using solution-processed method so as to investigate the effect of polyoxometalate molecular doping on photovoltaic performance. In virtue of polyoxometalate (H3PW12O40, denoted as PW12) doping in CZTS thin film, the photovoltaic performance of the solar cell is improved obviously, and a 26% increase in short circuit current density and 38% increase in power conversion efficiency can be attained. This is mainly attributed to the occurrence of photogenerated electron transfer from CZTS to POMs, so that the electron-hole recombination in CZTS is suppressed effectively. These results obtained in this work demonstrate that incorporating polyoxometalates into CZTS film can effectively improve the photovoltaic performance of CZTS thin film solar cells, although the simple solar cell device used as control experiment exhibits a very low efficiency. Thus the study provides a new strategy for boosting the photovoltaic performance of CZTS thin film solar cells by polyoxometalate molecular doping.