The invention discloses a subgrade strengthening structure for a highly karstified station turnout zone on a high-speed railway, which has the characteristics of high strength, high rigidity, high stability, high durability, simple construction process and great reduction in the construction cost. The subgrade strengthening structure comprises reinforced concrete cast-in-situ bored piles (4), reinforced concrete trimmer beams (5), reinforced concrete bearing sheets (6), earth bank bedding layers (8) and earth bank bedding surface layers (9), wherein the reinforced concrete cast-in-situ bored piles (4) are arranged at intervals along the transverse direction and longitudinal direction of a subgrade; pile bodies vertically pass through a subgrade soil body (1) to a position where the bottom of a karst cavity (3) in soluble bedrock (2) is greater than or equal to 0.5 meter; the reinforced concrete trimmer beams (5) are arranged at intervals on the tops of the reinforced concrete cast-in-situ bored piles (4), and are rigidly connected with the reinforced concrete cast-in-situ bored piles (4); the reinforced concrete bearing sheets (6) are arranged on the tops of the reinforced concrete trimmer beams (5) and the reinforced concrete cast-in-situ bored piles; the earth bank bedding layers (8) are filled on the reinforced concrete bearing sheets (6); and the earth bank bedding surface layers (9) are filled on the earth bank bedding layers (8).