Abstract: Flexible K-doped CZTSSe thin films were synthesized by selenizing the magnetron co-sputtered precursors. The influence of the K-content on the microstructures, energy band structures of the thin film and performance of the solar-cells was investigated with X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and Raman spectroscopy. The preliminary results show that the K-content had a major impact. For example, as the K-content increased, < 112 > preferentially-oriented growth became increasingly obvious, the grain-size changed in an increase-decrease mode; the over amount K-induced conduction band offset (CBO) increased and the CBO absolute-value at CZTSSe/CdS interface increased because K-doping induced band structure re-alignment, resulting in efficiency deterioration of solar-cell. With K-content of 1. 0 μmol, the < 112 > preferentially grown CZTSSe thin film had average grain-size of 1 μm and optimal CBO at CZTSSe/CdS interface. Moreover, the highest efficiency of the K-doped flexible solar-cell reached 3. 06%.