Low Voc is the major factor limiting the current efficiency of pure sulfide Cu2ZnSnS4 (CZTS) thin film solar cells. In this work, stacking two CZTS layers with different majority doping levels is employed to increase Voc. With this double-layer structure, the obtained highest efficiency is 7.57% with Voc of 734 mV, Jsc of 15.4 mA/cm(2) and FF of 67.0%. Compared to the traditional device with single CZTS layer (Efficiency: 7.62%, Voc: 665 mV, Jsc: 20.0 mA/cm(2), FF: 57.3%), the average Voc has been increased by 70-80 mV. However the Jsc was decreased due to voids within the CZTS and severe band tailing problem, limiting the further boost of efficiency for double-layer CZTS solar cell.