Leveraging the inherent advantages of silicon，flexible silicon photodetectors have become core components in flexible electronics applications，such as smart wearables，electronic skins，and foldable displays. However，performance evolution mechanisms of puresilicon flexible optoelectronic devices under mechanical deformation remain insufficiently explored. In this work，a multiphysics coupling model encompassing “optical-electrical-mechanical” fields is established to systematically investigate the spectral characteristics and photoelectric transient responses of siliconbipolar junction phototransistors(BJPTs)with varying active layer thicknesses(4.0~6.0 μm)under uniaxial tensile bending conditions(curvature radii ranging from flat to 10 mm). The results reveal a “thickness-strain” tradeoff between the photoelectric performance and mechanical fatigue resistance of the devices. Thicker devices of 6. 0 μm thickness，whose surfaces are located farther from the neutral mechanical plane，experience nonuniform strain gradients. Under bending at a 10 mm radius，the photocurrent decreases by 16. 5%，and the on/off ratio drops by 46. 0% owing to a 55. 1% increase in defect-induced dark current. In contrast，devices with an active layer thickness of 4. 0 μm feature geometries closer to the neutral mechanical plane and release macroscopic lattice stress. Consequently， their photocurrent attenuation is limited to within 6. 0%， and the specific detectivity is maintained at Jones. This study revealed the physical mechanisms by which designs of varying thickness suppress asymmetric band distortion and defect accumulation， providing theoretical support for the development of highly reliable， stress-drift-resistant pure silicon flexible optoelectronic sensing systems.