The effects of focusing on the measurement precision of radiometric pyrometer have been analyzed. The results show that the larger of the high temperature region, the larger of the permissible focusing range. In experiment such as laser-induced diffusion, the diameter of the exposed region is only tens of microns. Therefore, the requirement of focusing is stringent. Then a focusing method was proposed to solve this problem. In the system used in the experiment, the image distance is fixed, and the focal length of the object lens is 30 mm at wavelength of 0.546 μm. The calculated results suggested that the change of focal lengths at different wavelengths causes the change of objective distances obviously. Based on these calculated results and the measured temperature distributions of the same object at different objective distances, the objective distance can be adjusted in the range of ± 0.05 mm around the ideal value, to meet the need of the radiometric temperature measurement for a small region.