The displacement damage effects on indium gallium arsenide (InGaAs) p-i-n photodetectors induced by neutron at China Spallation Neutron Source (CSNS) are analyzed. The irradiation fluence range from 1.0 $\times$ 10¹¹ to 3.6 $\times$ 10¹³ cm${}^{-2}$. The InGaAs p-i-n photodetector under investigation was fabricated with top illumination structure. The forward and reverse bias current–voltage (I–V) under the dark environment is measured before and after neutron radiation in the room environment. Experiment results for different bias conditions and different irradiation flux were compared. The mechanism of displacement damage effects on the I–V characters of InGaAs p-i-n photodetectors are analyzed combined with TCAD simulation. The annealing effects on InGaAs p-i-n photodetectors are also investigated.