Infrared imaging is the concentrative embodiment of the highest level of contemporary infrared technology, and infrared imaging technology becomes one of the key technologies of current national defense science and technology. In the infrared imaging simulation, after gaining the temperature of object scene, firstly establish the radiation temperature model to calculate the radiation quantum of object, and then produce the infrared images according to the gray mapping model. In addition, when calculate object real temperature with the measured temperature from thermograph, temperature value need to anti-count with radiation temperature model. This paper gives an optimization algorithm by least-squares fitting in allusion to the calculating complexity by Planck formula. By comparing performance, it is proved that this algorithm can greatly reduce the calculating time in precondition of guaranteeing the accuracy.