Sulfite is effective in the waste activated sludge (WAS) pretreatment for enhancing its solubilization, disintegration, and biodegradability. Given the various structures and constituents of the WAS and primary sludge (PS), how sulfite pretreatment affects the characteristics of the PS remains unclear. This study is the first to consider the application of the sulfite pretreatment to a full-scale-plant-derived PS treatment to reveal the changes in sludge properties. The methane production from the PS, pretreated with 500 mg SO-S/L of sulfite, was reduced by up to 37%. However, the solubilization of the PS was enhanced by the sulfite pretreatment, leading to substantial substrates being released from the PS, namely soluble polysaccharides, by 96%, and soluble proteins, by 304%. Sulfite directly reduced the concentration of the aromatic amino acid tryptophan in the PS, while the functional groups of key substances in the PS were destroyed by sulfite. Moreover, the dewaterability and degradation extents of the PS were enhanced following the sulfite pretreatment with a low capillary suction time and high solid reduction. The findings of this study suggest that the sulfite-based technology is viable for PS pretreatment prior to the dewatering and final disposal stages towards efficient post sludge treatment.