Here, the present research situation of all inorganic thermometers based on fluorescence intensity ratio (FIR) technology is reviewed. The thermometers are classified in detail based on type of luminescence center, and the principle equations of the thermometers are derived. The results show that the temperature sensing principles of single emission center and dual emission centers are similar. Further, the dual emission centers thermometers are classified into four different types and their characteristics are analyzed. The performance parameters of the thermometer, absolute sensitivity, relative sensitivity, resolution and repeatability have been discussed, respectively. The analysis results of a large number of studies show that the sensitivity is affected by the matrix phonon energy, crystal coordination environment, material size, doping concentration, etc. Inorganic optical thermometers show great potential in non-contact temperature sensing due to the excellent repeatability of inorganic materials . We summarize the current difficulties and look forward to the future of thermometers. Therefore, the review has positive effect on the development of inorganic FIR thermometers towards excellent performance.