The reductive gas composition produced by the reburning of sludge can reduce NOx in flue gases to N2, which provides a new idea for the study of gas denitration, and at the same time makes denitration by sludge reburning become an important part of gas denitration research. However, due to the complexity of the sludge structure, the existence of reductive groups in the sludge is still unclear. This study aims at exploring these highlighted problems. First, the thermal decomposition of sludge was analyzed at five gradient temperatures. Then, the products of the thermal decomposition at the various temperatures were analyzed by nuclear magnetic resonance (NMR). Finally, the forms in which the reductive groups in exist sludge were deduced using the inversion method, backward from higher to lower temperatures. The results of the experiment indicate that the reductive groups in the sludge exist in the form of organic functional groups or hydrocarbon such as CH3N=, RCH2–S–, RCH2–N=, RCH2–O–, R2CH–N=, R2CH–S–, and cycloalkynes. The NMR inversion method employed contributed to the prevention of the superposition of NMR peaks caused by the complex structure of sludge and improved the accuracy of the analysis.