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• Precursor-heating strategy is introduced to tailor the stress of Cu 2 ZnSn(S,Se) 4. • 100℃ treated precursor contributes maximum stress reduction in Cu 2 ZnSn(S,Se) 4. • Decreased porosity contributes improved mechanical strength of Cu 2 ZnSn(S,Se) 4. • Precursor-treated device exhibits more satisfying bending stability. Co-existing feature with high efficiency and desirable mechanical tolerance for flexible solar cells plays a decisive role in potential industrialization. In this study, we demonstrated a precursor-heating strategy in fabricating flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cell, which effectively decreased residual stress, enhanced mechanical strength and reduced carrier recombination of flexible CZTSSe solar cells. Compared to strategy without precursor-heating, the prepared CZTSSe thin film with 100 °C precursor treatment exhibited a decreased residual stress from −3.39 GPa to −0.71 GPa, featuring an increased efficiency from 3.07% to 4.40% and satisfactory mechanical stability even suffered from various harsh bending. The work paves a forward way for future large-scale application in portable devices. [ABSTRACT FROM AUTHOR]