With the rapid development of the sewage treatment, dewatered sludge, as an inevitable byproduct, is increasing its amount. Anaerobic digestion performance of dewatered sludge flocculated by polyacrylamide under different biopretreatments conditions were studied. The feeding concentration was 3% of total solid, and the digestion temperature was 35℃. The cumulative biogas yield, methane content, pH value, ammonia nitrogen content and total chemical oxygen demand (TCOD) were analyzed to explore the effect of fungus—Paecilomyces variotii on anaerobic digestion process. The result showed that biopretreatment of dewatered sludge prior to anaerobic digestion was an alternative method. Direct addition and 2 d biopretreatments can both enhance cumulative biogas yield and methane content with faster hydrolysis rate and better organic matter degradation rate. Compared with the pure sludge treatment, direct addition of Paecilomyces variotii was optimal, and its cumulative biogas production and methane production were increased by 85.79% and 42.76%, respectively. While 1 kg dewatered sludge can produce 12.69 L methane with an increase of 42.74% of sludge without pretreatment. The result would provide theoretical basis for solving the problem of dewatered sludge resourceful treatment.