The invention relates to a combustion stability determining method based on thermal decomposition. The pulverized coal combustion stability determination condition is acquired by the raw coal thermal decomposition method, the boiler design parameters are combined additionally, and the combustion stability of one power plant boiler is predicted previously. Compared with the prior art, the method has the advantages that the target of preventing and intervening power plant boiler blended combustion previously is achieved, the hysteresis and randomness caused by real-time monitoring are avoided, the full-load prediction of pulverized coal combustion stability can be implemented, and the applicability of combustion stability determination is improved.