Sulfite has been widely employed as a key agent in many industrial processes, leading to a large amount of sulfite-laden wastes generated. Given its antimicrobial function and destructive ability on cell walls, detailed mechanisms for impacts of sulfite on waste activated sludge (WAS) and outcomes of methane production after the sulfite-pretreatment have not been clear so far. In this study, the feasibility of methane production from sulfite pretreated WAS was verified and investigated. Biochemical methane potential tests demonstrated that methane production from WAS after the pretreatment at 800 mg S/L of sulfite (a typical level in sulfite-laden wastes) increased by up to 25%. Kinetic analysis of the test results indicated that sulfite pretreatment increased the sludge hydrolysis rate (k ) by 1.71 times while the ultimate biochemical methane potential (B ) by 1.20 times. Further study investigated the effects of sulfite on WAS from the macro-scale (i.e. sludge physicochemical properties) to the micro-scale (i.e. bacterial viability, microbial community). Sulfite concentrations of up to 800 mg S/L substantially enhanced WAS disintegration and solubilization, reducing the particle size by up to 39%, boosting substrate release by 87% and improving cell lysis by 43% through the direct destruction of gram-positive microorganisms (e.g., norank_p_Saccharibacteria) in WAS. Adverse impact on anaerobic digestion by introduction of sulfite was not observed in this study, though a long-term evaluation is needed in the future work. Based on the findings of the present study, sulfite-laden by-products or wastes from industrial processes may be co-treated with WAS when overall cost-effectiveness is concerned.