Cu2ZnSn(S,Se)(4) (CZTSSe) films with smooth surface were prepared by solution approach and etched (etched-CZTSSe) through using KMnO4 in a H2SO4 based medium followed by Na2S. Two types of solar cells with conventional structure were fabricated with CZTSSe and etched-CZTSSe as absorber, respectively. It is demonstrated by XRD, XPS, EDS and Raman measurement that the bulk CZTSSe is of kesterite structure while its surface contains a small amount of Cu2ZnSn3Se8 and ZnSe secondary phases besides kesterite CZTSSe. On the surface, the Cu2ZnSn3Se8 is completely removed after etching while the ZnSe is partially. It is found that the elimination or reduction of the secondary phases increase the shunt resistance greatly, leading to a larger open circuit voltage (V-oc) which results in the improvement of power conversion efficiency (PCE) for the etched-CZTSSe-based solar cell. The PCE has a maximum increase of 26% by etching. An intensive study has been made for the influencing mechanism of etching on the PCE of solar cells.