In order to research the performance of the laser on a focal plane array thermal imager in a wavelenth,several field range irradiation experiments were designed and the actual light path transformation of the laser beam was calculated and simulated with the corresponding beam path theoretical calculation model. In theoretical analysis,the flat-topping laser beam, optical stop formed by a laser output mirror and the optical elements in the light path were simulated by the flat-topping Gaussian function and the sum of a finite number of complex Gaussian functions, then the delivery and transformation of laser beam were calculated with the Collins diffraction formula of a paraxial optical system.In the laboratory, the laser beam disturbance on the thermal imager was implemented by placing the singlet and changing the distance between the optical element and the laser. Experimental results indicates that low power CW lasers could disturb the thermal imager and the disc of confusion on the focal plane could overlay almost approximate 30 percent of image cells.The conclusion shows that the theoretical calculation is coinciden with the experimental results well.