This study proposed a novel free ammonia (FA, i.e., NH3) pretreatment technology to enhance anaerobic methane production from primary sludge for the first time. The solubilization of primary sludge was substantially enhanced following 24 h FA pretreatment (250–680 mg NH3-N/L), by which the release of soluble chemical oxygen demand (SCOD) (i.e., 0.4 mg SCOD/mg VS added; VS: volatile solids) was approximately 10 times as much as that without pretreatment (i.e., 0.03 mg SCOD/mg VS added). Then, biochemical methane potential (BMP) tests demonstrated that FA pretreatment of 250–680 mg NH3-N/L was capable of enhancing anaerobic methane production while the digestion time was more than 7 days. Model based analysis indicated that the improved anaerobic methane production was due to an increased biochemical methane potential (B0) of 8–17% (i.e., from 331 to 357–387 L CH4/kg VS added), with the highest B0 achieved at 420 mg NH3-N/L pretreatment. However, FA pretreatment of 250–680 mg NH3-N/L decreased hydrolysis rate (k) by 24–38% compared with control (i.e., from 0.29 d−1 to 0.18–0.22 d−1), which explained the lower methane production over the first 7 days’ digestion period. Economic analysis and environmental evaluation demonstrated that FA pretreatment technology was environmentally friendly and economically favorable. Biotechnol.