To study the jamming effects of laser on CMOS image sensor, jamming experiments with 632.8 nm CW laser were conducted. With the increasing of laser power, the phenomenon of instauration, saturation and full screen saturation were observed. Pixel upset effect in the maximum laser intensity site of the irradiated CMOS sensor was discovered when the intensity was greater than 1.4 W/cm2 and the full screen saturation had not appeared at this time. Even the laser intensity at the photosensitive surface was up to 95.1 W/cm2, the camera could still image normally when the laser irradiation was terminated. It indicated that the pixel upset effect was not caused by laser damage. Analysis based on the device structure, signal detection and processing of the sensor chip was then carried out. It shows that one possible reason is that the two measurements of pixel output voltage(Vsignal and Vreset) are gradually approaching to each other for the electric charges of photodiode capacitance will be quickly reduced by excessive photo-carriers, and the Vreset will be subtracted from Vsignal in correlated double sampling.