In order to explore the appropriate addition ratio of poly-β-hydroxybutyric acid (PHB) as a slow-release carbon source in biological flocculation systems, PHB was added to 3 trial groups respectively at the ratio of 80 g/m3 (group A), 200g/m3 (group B) and 320 g/m3 (group C), and a 48-day breeding experiment was conducted. Water quality monitoring was carried out every 4 days. At the end of the experiment, the growth of the fries was detected, the flocs were taken for nutrient index detection, and the PHB was weighed for scanning electron microscopy and infrared pectroscopy. The research results showed that with the increase of PHB addition, for all groups, the peak value of total ammonia nitrogen (TAN) gradually decreased, and the accumulation level of nitrate nitrogen (NO3—N) increased. In the later stage of the experiment, group A could stably maintain the system DOC/TN at a level above 15. Different PHB additions had no significant effect on the survival rate and weight gain rate of the cultured tilapia (P>0.05), and the feed coefficient of group C was significantly lower than that of group A and group B (P<0.05). Different addition amounts of PHB as a system carbon source showed significant differences in degradation loss (P<0.05). With the increase of PHB addition, the higher the degradation loss, the lower the degree of surface degradation and damage. In summary, a PHB addition of 80 g/m3 could maintain a high level of system C/N above 15 during the breeding period, and could be used as a suitable addition ratio for PHB as a slow-release carbon source in biological flocculation systems.