The crystallization process plays a decisive role in fabricating efficient and flexible Cu₂ZnSn(S, Se)₄ (CZTSSe) thin film solar cells (TFSCs). However, the unknown difference of crystallization kinetics of CZTSSe films by different solution systems remains to be distinguished for efficient and flexible CZTSSe TFSCs. In this work, based on mainstream amine-thiol (AT) and 2-methoxyethanol (C₃H₈O₂, MOE) solution systems, the crystallization kinetics of CZTSSe films and the photoelectronic properties of relevant flexible devices are well compared and studied. The results show that AT solution processed CZTSSe films form a bi-layer structure with bottom small grains under unidirectional grain growth mode, whereas there are large CZTSSe grains throughout the MOE solution processed CZTSSe film under bidirectional grain growth mode. In addition, significant composition deviation, undesirable band gap alignment, and carbon residues except for excellent flexibility and mechanical durability can be found in CZTSSe-AT films, while CZTSSe-MOE films possess well compositional uniformity, desirable band gap alignment and consistency with precursor solution. Finally, better heterojunction quality, lower interfacial defects concentration, free of direct carrier recombination path, smaller quasi neutral region width and fewer copper vacancy (V_Cu) defects lead to an evident increase of the short-circuit current density (J_SC) for TFSC-MOE by 14.26%, which demonstrates a better carrier transportation and extraction ability. Meanwhile, the power conversion efficiency of MOE processed flexible CZTSSe TFSCs (9.92%) is enhanced by 8% compared to that of AT processed ones (9.18%). These results can offer a deeper understanding on crystallization kinetics of CZTSSe films, and offer solid theoretical reference for future efficient flexible CZTSSe TFSCs.