In this study, polylactic acid (PLA) and poly3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) were used as the external carbon source to explore release rule of the carbon source of PLA and PHBV under high, medium and low salinity in simulated seawater biofloc cultivation. The release rule is related to its influence on the water quality, microbial diversity and community structure in the aquaculture of marine biofloc. The results show that: PHBV carbon source is better than PLA carbon source, medium salinity is more conducive to the conversion of various nutrients. The mass concentration of salt and ammonia nitrogen is finally kept between 3.22~6.23 mg/L, and the phosphate is finally kept below 0.01 mg/L. In terms of the establishment of the biofloc system,the lower the salinity is, the faster the biofloc system is established. In the early stage of the experiment, in the low-salinity system,  PLA and PHBV had the lowest environmental average ammonia nitrogen concentration, which were 12.82 mg/L and 8.08 mg/L, respectively. The establishment of the biofloc system of the PHBV carbon source group was faster than that of the PLA carbon source group. In the establishment of microbial community, in the high-salinity system, Donghicola and Bacillus will cooperate to form a stable system. The study has shown that both PLA and PHBV carbon sources can be used in marine biofloc culture, and PHBV has a better effect.