Abstract: Taking the high moisture-containing sewage sludge (SS), herbal tea waste (HTW) and diatom (DT) as the feedstock, the characteristics of NO_x precursors during pyrolysis with or without hydrothermal pretreatment in a horizontal tubular reactor were compared. The formation mechanism of NO_x precursors in pyrolysis coupled with hydrothermal pretreatment was also investigated by means of TGA and XPS techniques. The results show that the hydrothermal pretreatment can affect the formation pathways related to NO_x precursors at different pyrolysis stages and reduce the release amount of NO_x precursor on the whole level. For example, when the pyrolysis temperature is 900 ℃, the NO_x precursor yield derived from the hydrothermal treated coke is 55.0% for SS₂₄₀, 48.1% for HTW₂₄₀ and 51.2% for DT₂₄₀, which is 9.5%, 6.0% and 15.4% less than that for SS, HTW and DT untreated sample, respectively. But if calculating based on the amount of N content in feedstock, the released NO_x precursor from the hydrothermal treated coke is 90.1%, 41.9% and 59.8% less than that for SS, HTW and DT untreated sample, respectively. The inhibition effect on NH₃ formation is higher than that on HCN formation. Meanwhile, two influencing pathways caused by hydrothermal pretreatment were further elaborated, i.e., the removal of N functionalities that leads to a decrease in NH₃ on the primary reaction and the stabilization of N functionalities that leads to a decrease in HCN on the secondary reaction.