Abstract: Jamming effect of laser on IR imager is an interesting research subject in electro-optical countermeasure. Jamming experiment and analysis were conducted. A pulsed CO2 laser with the wavelength of 10.6 m was used, and the laser was reflected to a polysilicon 320240 uncooled focal plane array (UFPA) infrared thermal imager through a diffuse plate. By adjusting the laser energy density, frequency, wavelength and the size of laser spot, as well as analyzing the jamming image quality, the influence of laser parameters on jamming effect was studied. The results show that the jamming effect is improved with the increasing of energy, frequency, wavelength and size of laser spot. Moreover, the jamming effect proves the feasibility of the diffuseness jamming, which avoids the interference source becoming the bait of the laser anti-radiation missiles and provides a new tactical solution for laser jamming.