CZTSSe (sulfur around 1%) absorber was prepared by sputtering on a CZTS target followed by selenization under Ar carried H2Se gas. The mechanism of CZTSSe formation was investigated by Ar annealing and selenization of sputtered CZTS precursor. The apparent decomposition temperature of CZTS precursor annealed under Ar atmosphere was between 450 degrees C and 500 degrees C. While the temperature that CZTS precursor transformed into high-selenium CZTSSe film annealed under Ar carried H2Se gas was determined to be <= 400 degrees C. CZTSSe with single grain layer was obtained when selenization temperature exceeded 520 degrees C. By supplying H2Se from suitable temperature, the metal components of precursor and resulting absorber were almost the same. The highest efficiency of 9.6% achieved at the optimal temperature of 560 degrees C.