Based on the basic theory of infrared radiation, infrared tri-wavelength vectors group altering low coupling between emissivity and infrared temperature measurement was derived to isolate the object emissivity and infrared temperature measurement. In order to optimize the tri-wavelength vectors group, a non-dimensional emissivity model was established and the most suitable wavelength range was identified. Aiming at optimizing the linear function of emissivity, the bending index n was introduced. The appropriate wavelength range to guarantee exactness of result of tri-wavelength vectors group was presented. The tri-wavelength measurement was demonstrated in a laboratory experiment with four sulfide ore samples. The results show that the fitting of tri-wavelength measurement with true temperature is better than that of common infrared temperature measurement. The fractional error of tri-wavelength measurement significantly is less than the common. The availability and accuracy of tri-wavelength measurement are verified. The tri-wavelength measurement ensures the measurement accuracy as well as expands the scope of common thermal infrared imager application and ensures the numerical accuracy of infrared prediction in the spontaneous combustion of sulfide ores.