Raising the carbon/nitrogen ratios of Biofloc technology(BFT) aquaculture system by providing carbohydrates can promote the growth of heterotrophic bacteria, and reform bioflocs with feces and lures in cultured water, so as to improve the utilization rate of feed protein and purify the aquaculture water. However, there is no uniform view on how to add carbohydrates. In order to prevent the aquaculture animals from being poisoned by ammonia nitrogen and nitrite nitrogen during the construction of BFT system, it is necessary to set the start-up stage of the BFT system. Therefore, a 60 days experiment was carried out to investigate three strategies of carbohydrates providing at the start-up stage of the biofloc system with salinity of 30 ppt, aiming at optimizing the adding strategy of glucose, so as to shorten the formation period of the biofloc system and stabilize the water quality quickly. The first method is to add glucose to the biofloc technology (BFT)system (group A) at a one-time to make the C/TN (total nitrogen) be 15(group A). The second is 10% total glucose of group A was added every day from 1st to 10th day, after that, if the total ammonia nitrogen (TAN) increased to above 1 mg/L, the glucose was added according to C/TAN of 6. The third is to add glucose every day according to C/TAN of 6. The results showed TAN in A and C treatment groups was generally at a low level during the experiment period. TAN in B group reached a peak value of (16.36±3.02)mg/L on the 31st days, nitrite nitrogen(NO₂^--N) and nitrate nitrogen(NO₃^--N) had a significant accumulation, but dropped to the lowest level on the 59th day. The three treatment groups both had assimilation and nitrification, but the nitrification was obviously inhibited in the system with continuous addition of carbohydrates. The extracellular polymeric substances (EPS) and crude protein of bioflcos in the three treatment groups showed the trend of decreasing during the experiment period, and the settling velocity of bioflocs of the three treatment groups becomes better. The bacterial community of bioflocs was analyzed by high-throughput sequencing. The results showed that, at the phylum level the main dominant microflora of the 3 different groups were Proteobacteria. Besides,Bacteroidetes and Planctomycetes were also found in group C. At the class level, the main advantage microflora in group A and group B were mainly Gammaproteobacteria, while the main dominant microflora in group C are Gammaproteobacteria,Alphaproteobacteria, Flavobacteria and Phycisphaerae. Thus, continuous addition of carbon sources can enrich microbial species. The experiment shows that the three carbohydrates additions can stabilize the water quality in about 55 days in a seawater aquaculture system using bioflocs technology.It can be concluded that the best method for the start-up of seawater BFT system is maintaining C/N 15 initially, then adding carbon source to 6 of C/N when TAN was up to 1 mg/L.