Pinning voltage (Vpin) is the key parameter affecting the charge transfer efficiency and full well capacity of the pinned photodiode (PPD) in CMOS image sensor (CIS). Experimental results show that Vpin increases with the total radiation dose (TID) effect, so it is of great significance to study the reasons in the design of anti-irradiation CIS. In this paper, TCAD simulation software is employed to analyze the electrical characteristics of the CIS and study the mechanism of Vpin affected by TID. The results show that when the concentration of trapped charges caused by irradiation reaches 3×1016cm-3, the depletion region near the shallow trench isolation (STI) isolates the pin layer of the PPD from ground, which causes the potential of the pin layer to be easily affected by the transfer transistor channel to increase. Thus under the same electron injection, the number of electrons stored in the PPD increases and the reset voltage needs to be higher, and Vpin increases with the increases of TID.