Taking 4T PPD (4 transistor clamped photodiodes) type CMOS image sensor as the research object, the damage simulation of neutron irradiation fluence of 1×10 ¹¹, 3×10 ¹¹, 5×10 ¹¹, 7×10 ¹¹, 1×10 ¹² neutron/cm ² are carried out, and the device model of CMOS image sensor and the defect model of displacement damage after different neutron irradiation fluence are established. The correlation double sampling technique is used to measure the output value of floating diffusion (FD) under two continuous pulses from bright to dark, and a simulation method for measuring charge transfer loss (CTI) is established. The relationship between CTI and neutron irradiation fluence is obtained, and the variation of CTI with neutron cumulative fluence is analyzed. Combined with neutron irradiation effect experiments to verify the validity of the theoretical simulation results of neutron irradiation-induced CTI degradation. The results show that the sensitive region of displacement damage of CMOS image sensor is space charge region, and displacement damage defects will be introduced into space charge region after neutron irradiation. Through continuous capture and emission of carriers, these defects make the signal charge can not be quickly transferred to FD, resulting in charge transfer loss, and the charge transfer loss increases with the increase of neutron irradiation fluence, and there is a linear relationship between them in a certain range.