The invention provides a leakage prevention construction method of a foundation pit support structure under a blasting condition. The leakage prevention construction method comprises the following steps: (1) a geological condition of a construction site is determined; (2) a soil covering body on a rock surface is excavated away; and the grouting treatment is performed for a generated wall crack; (3) a rock above the depth of a dig foundation pit is controlled to blast; (4) the crack is blocked by a method of combining injection of a double-component polyurethane high polymer material with high--pressure jet grouting pile formation; (5) blasting parameters are optimized; and (6) a next unit of blasting area is continuously blasted until the layer of blasting area finishes blasting; and after the layer of blasting construction is finished, the construction of a next layer of blasting area is performed. The leakage prevention construction method can effectively block the crack of the support structure to close an underground water leakage path on the outer side of the foundation pit, optimizes the blasting parameters step by step, and reduces the influence on safety of the support structure from blasting construction.