When employing an infrared thermal imager to measure surface temperature, the emissivity of the surface to be measured should be set first and kept constant during the measurement process. However, when the infrared imager is placed in the range of more than 50. of the zenith angle of the points to be measured, the emissivities of the points in this angle will vary significantly; hence, temperature measurement errors will occur, especially for points on non-flat surfaces. In view of the measurement error caused by the variation of emissivity of different points in the measured non-flat surface when using a monocular infrared thermal imager, this paper provides a compensation factor based on the variation rules of the emissivity with the measuring angle. In addition, based on 3D modeling technology, the relationship between the positions of the points in the thermographic image and those in the actual surface is determined. The compensation method of the temperature measurements for a non-flat surface is presented. The feasibility of the method was verified through experiments.