The principles of the working and temperature measurement for the infrared thermal imager are introduced. The factors that influence the measurement precision of the infrared thermal imager are analyzed. The mathematical expression that the error factors influence the temperature measurement is derived in theory, and the error curve under different temperature conditions for the materials with different emissivity are calculated. The temperature measurement precisions of middle infrared (MIR) and long infrared (LIR) thermal imager are compared.