Polyhydroxyalkanoate (PHA) is a type of biodegradable plastics. The synthesis of PHA by activated sludge has the advantage of easy operation. With acetic acid as the carbon source, the anaerobic synthesis of PHA in the anaerobic-aerobic activated sludge system was compared with the aerobic one in the aerobic sludge system. It was observed that the aerobic PHA synthesis was greater than the anaerobic one. The influences of acetic acid addition frequency, acetic acid concentration and allyl thiourea (ATU) on aerobic PHA synthesis were further investigated. The results showed that the PHA content accounting for volatile suspended solids reached 56.3% under conditions of acetic acid addition with frequency of 3, acetic acid concentration of 2 925 mg/L, and ATU concentration of 6.74 mg/L.