Hydroxypropyl cellulose is added into the precursor solution to deposit Cu2ZnSnS4 (CZTS) thin film for super-strate solar cell. The crystallinity and morphologies of CZTS thin films can be manipulated by adding a small amount of hydroxypropyl cellulose. With addition of 1.0 wt% hydroxypropyl cellulose, the crystallinity and grain size of CZTS are obviously improved, and the light-to-electric conversion efficiency of CZTS thin film solar cell is enhanced from 2.07% to 3.92%. The photoelectric performance enhancement mechanism is analyzed by the measurements of electrochemical impedance spectroscopy and intensity modulating photoelectrochemical spectroscopy of CZTS thin film solar cells and certificated to be formation of the bigger CZTS grains due to addition of hydroxypropyl cellulose, which reduces the boundary defects and makes the film surface be more continuous. These results suggest that addition of hydroxypropyl cellulose into the precursor solution is an effective way to obtain high performance CZTS thin film solar cells.