Understanding the stress-induced phenomena is essential for improving the long-term application of flexible solar cells to non-flat surfaces. Here, we investigated the electronic band structure and carrier transport mechanism of Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic devices under mechanical stress. Highly efficient flexible CZTSSe devices were fabricated controlling the Na incorporation. The electronic structure of CZTSSe was deformed with stress as the band gap, valence band edge, and work function changed. Electrical properties of the bent CZTSSe surface were probed by Kelvin probe force microscopy and the CZTSSe with Na showed less degraded carrier transport compared to the CZTSSe without Na. The local open-circuit voltage (VOC) on the bent CZTSSe surface decreased due to limited carrier excitation. The reduction of local VOC occurred larger with convex bending than in concave bending, which is consistent with the degradation of device parameters. This study paves the way for understanding the stress-induced optoelectronic changes in flexible photovoltaic devices.