In this paper, we discuss the dark signal increase in complementary metal oxide semiconductor (CMOS) active pixel sensor due to proton-induced damage, and present the basic mechanism that may cause failure. When the fluence of protons reaches a predetermined point, the change of dark signal of the device is measured offline. The experimental result shows that as the fluence of protons increases, mean dark signal increases rapidly. The main reason for dark signal degradation is: 1) the ionizing damage causes a build-up of oxide trapped charge and interface state at the Si-SiO2 interface. The creation of the interface traps (with energy levels within the silicon bandgap), which can communicate with carriers in the silicon, gives rise to the thermal generation of the electron-hole pairs and, hence increasing the dark signals; 2) when protons pass through the sensor, there is a possibility of collisions with silicon lattice atoms in the bulk silicon. In these collisions, atoms can be displaced from their lattice sites and defects are formed. These resulting defects can give rise to states with energy levels within the forbidden bandgap. The increasing of dark signal is therefore one of the prominent consequences of bulk displacement. We use multi-layered shielding simulation software to calculate the ionization damage dose and displacement damage dose. Based on the comparison of the test data of gamma radiation, combined with the device structure and process parameters, a theoretical model for separation proton-induced ionization and displacement damage effects on CMOS active pixel is constructed, and the degradation mechanism of the mean dark signal is investigated. The result shows that the contribution of ionization effect induced surface dark signal and the contribution of displacement damage induced bulk dark signal to dark signal degradation of the whole device are roughly equal in this domestic CMOS active pixel.