A method for constructing a high-strength cavity at the bottom of a vertical well is used for exploiting natural gas hydrates in sea areas, and comprises the following construction steps that drillinghydrate overlying strata; drilling a stratum below a hydrate bottom plate; expanding a cavity at the stratum under the hydrate bottom plate; pouring high-pressure cement paste without lifting a drillto form a high-strength cement primary pile; drilling the high-strength cement primary pile; secondary cavity expanding; removing slag in the cavity; cavity expanding detection; cementing and filling; and completing the vertical well bottom cavity construction. A reaming cast-in-place pile technology is adopted, by means of two-time cavity expanding and two-time pouring pile forming, the diameterof the primary pile is large, the final pile is subjected to cavity expanding pouring on the basis of the primary pile, the final pile is high in strength and regular in shape, the vertical well bottom cavity is built on the final pile, the stratum strength around the cavity is high, and the requirement on gathering gas production from multiple well groups of horizontal wells to the vertical wellcan be met.