The invention relates to a method utilizing buoyant rafts to breed big oysters. The method comprises the following steps that 1, an offshore sea area with the water depth of 10-20 meters, the salinityof 0.9% or below, the lowest water temperature of 5 DEG C or above and the dissolved oxygen greater than 5 mm / L is selected as a breeding sea area, and buoyant rafts are erected in the breeding sea area; 2, attachment bases where big young oysters attach are suspended under the buoyant rafts by suing ropes of 1-4 meters, and the attachment bases are suspended on the ropes at intervals to be immersed in seawater; 3, the total breeding time is three years, big young oysters with the length of 10-20 mm are suspended for breeding in the first year, the buoyant rafts are fixed in the sea area withthe salinity of 0.9%-1.5% for culture, the buoyant rafts are transferred and fixed to the sea area with the salinity of 1.5%-2% for culture in the second year, and the buoyant rafts are transferred and fixed to the sea area with the salinity of 2-3% for culture in the third year, and big oysters can be collected after the body length of the big oysters exceed 10 cm. By adopting the method, sufficient nutrients can be provided in the breeding process, and environmental destruction can be also reduced.