To investigate the impact of polyacrylamide (PAM) on hydrolysis, acidification, and microbial properties during anaerobic fermentation of waste activated sludge (WAS), the fermentation system was supplemented with four different concentrations of PAM. Results showed that PAM inhibited WAS cell disruption, which reduced volatile fatty acid (VFA) production but increased the proportions of propionic acid. The addition of PAM reduced protease and α-glucosidase activities by 41.52-75.58% and 21.7-41.63%, respectively, thus inhibiting the breakdown of organic substrates. Microbiologically, PAM reduced microbial richness and diversity, but increased the relative abundances of Bacteroidota and Actinobacteriota, which promoted the production of propionic acid. Additionally, PAM lowered the levels of key hydrolytic and acidogenic enzymes. This study offers further theoretical insights into how PAM influences VFA production during anaerobic sludge fermentation.