The device performance of Cu2ZnSn(S,Se)(4) (CZTSSe) solar cells are improved significantly by annealing the full devices at 270 degrees C for 3 min in air. This paper proposes the origin of the improvement of solar cell by post annealing treatment by analyzing the results of external quantum efficiency (EQE), Raman spectra, capacitance voltage (C-V), admittance spectra (AS) measurements, and numerical simulation. The experimental results show that the dominant free hole carriers changed from deep defect Cu-zn to the much higher density of shallow defect V-cu after annealing process, which resulting the increase of density of benign [V-cu + Zn-cu] defect cluster, and the decrease of density of detrimental [2Cu(Zn) + Sn-Zn] defect cluster in the surface region of absorber. As a result, the open voltage (V-oc) and fill factor (FF) increase significantly. The short circuit current density (J(sc)) increases slightly, which can be attributed to the decrease of interface recombination as well as the improvement of band alignment at CdS/CZTSSe interface. As a consequence, the conversion efficiency of CZTSSe solar cell is improved from 4.41% to 8.02% by post-annealing the full device. Additionally, numerical simulations using wxAMPS software are conducted to reveal insight into the role of the changes of defect type, defect density, and carrier density by post-annealing treatment.