Sodium dithionite and thiourea dioxide are two typical reducing agents commonly used in industry. This paper compared the thermal hazards of sodium dithionite and thiourea dioxide based on DSC-TG (Differential Scanning Calorimetry and Thermogravimetry) thermal analysis, small-scale self-heating experiments and FTIR smoke gas analysis. From the thermal analysis (DSC-TG), sodium dithionite starts to lose crystal water at the temperature around 60 °C which makes the sodium dithionite very unstable at relatively lower temperature. The decomposition of sodium dithionite has a close correlation with oxygen while the decomposition of thiourea dioxide has little relationship with oxygen. Small-scale self-heating experiments were designed to reflect the self-heating and spontaneous combustion properties of the characteristic dimension 20 cm. The small-scale self-heating experiments show that there is a little gap between the Critical Ambient Temperature (CAT) of sodium dithionite and thiourea dioxide. From FTIR smoke gas analysis, the smoke of thiourea dioxide (decomposition or combustion) is far more dangerous than sodium dithionite. Although the risk of spontaneous combustion for sodium dithionite is higher than thiourea dioxide, the thermal hazards of thiourea dioxide are far higher than sodium dithionite as the decomposition and combustion of thiourea dioxide would release far more heat and dangerous smokes.