The invention discloses an outdoor large-scale spat intermediate culture method. The method comprises the following steps that 1) an outdoor shellfish bed is constructed in a pond; 2) a water level height difference is gradually formed among a higher-place pond, an algae pond and the outdoor shellfish bed in the vertical direction, high-density shrimp culture is carried out in the higher-place pond, fish and shrimp mixed culture is carried out in the algae pond, and intermediate culture on spats is carried out in the outdoor shellfish bed; and 3) culture wastewater is purified: the culture wastewater in the higher-place pond is discharged into the algae pond for ectopic dilution and conversion firstly, the wastewater is changed into fertilizer water to promote the growth of beneficial unicellular algae, then the fertilizer water in the algae pond is introduced into the outdoor shellfish bed, the fertilizer water is changed into purified water through the filter feeding activity of bivalve shellfish, and meanwhile, the unicellular algae are used as bait organisms to promote the growth of the spats, so that the culture wastewater is purified in an ex-situ manner. According to the method, the breeding cost is greatly reduced, the productivity of intermediate culture of the spats is greatly improved, high economic benefits are achieved, the problem of treatment of culture wastewater can be effectively solved, and the method is suitable for popularization and application in areas suitable for breeding of bivalve spats such as clams and razor clams.