Abstract: Passive techniques for combustion diagnostics utilize flame spontaneous emission radiation information. The characteristics of this technique are noninvasive, low requirement on measurement environment, compact system, and easy to implement, and therefore it has unique advantages for online measurement of the combustion field. Firstly, the advantages and limitations of various combustion diagnosis technologies have been analyzed. Then, based on the work of passive combustion measurement diagnosis carried out by State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, the principles and state of the art of spontaneous emission radiation diagnostic are introduced according to the three aspects of flame emission spectroscopy, flame image processing, and thermal radiative imaging. Qualitative analysis of the combustion status and quantitative calculation of key combustion information such as the temperature and the species volume fraction in the combustion field can be achieved by using the three techniques above. Finally, the development trend of the spontaneous emission radiation diagnosis technology is presented, which is to obtain more plentiful measured signals, higher measurement resolution and accuracy, and more kinds of measured results.