The change of object distance would enormously decrease the precision of temperature measurementusing infrared thermal imaging system.In this paper, on the basis of theories of infrared object radiation andtranslationofradiation in geometry optics，formulas of tolerance of temperature measurement relations with object distance. Tolerance oftemperature measurement was calculated by using the parameters of a designed infrared thermal imaging system.Basing on the analysis，a method of improving the precision of temperature measurement by Matlab was given. The tolerance wouldbe just 0.09% to the same situation if the method was taken，which could reduce the effect on the precision of temperaturemeasurement using infrared thermal imaging system with the change of object distance.