This work studies degradation behavior after hot-carrier stress of trigate polysilicon thin-film transistors (poly-Si TFTs) with nanowires. The NH3 plasma passivation effect is also studied on the electrical characteristics after hot-carrier stress. The reliability of poly-Si TFTs with NH3 plasma passivation outperforms that without such passivation, resulting from the effective hydrogen passivation of the grain-boundary dangling bonds, and the pileup of nitrogen at the SiO2/poly-Si interface. The reliability of poly-Si TFTs further improves by using nanowires structure. These findings originate from the fact that the nanowires poly-Si TFT has robust trigate control to reduce the hot-carrier effect due to declining lateral electrical field and its penetration from the drain, and its split nanowire structure has superior NH3 plasma passivation effect. In degradation results under dc and ac stress, it reveals that the NH3 plasma is mostly passivated on deep traps of grain boundaries rather than tail traps.