The effects of the uniaxial mechanical strain stress on hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) were studied in this work. The proposed a-Si TFTs were fabricated on thin steel foil, and all process temperature was well-controlled below 200°C. The threshold voltage (V th) metastability was discussed by applying DC bias stress on gate electrode. The tensile strain was imposed on the device parallel to the source-drain current path, and all electrical parameters were extracted from saturation region. Our results indicated both outward and inward strain stress can lead to an un-recoverable destruction on a-Si:H TFTs at the first time. Even if the TFTs devices were re-flattened to plane, the transfer characteristic didn't recover and had larger V th variation than the consequent bending performance. This phenomenon was related to the disorder of amorphous silicon structure. We provide a model and used activation energy to explain this effect.