The invention discloses a variable-diameter pile cap structure of a foundation pile for an offshore building, which comprises a pile cap cylinder body, wherein an inner support mechanism for supporting the inner side surface of a pile body is arranged in the pile cap cylinder body; an annular base is arranged at the lower part of the pile cap cylinder body; at least three diameter-variable mechanisms for supporting the outer side surface of the pile body are uniformly distributed on the base in the circumferential direction; an avoidance through hole is arranged in the side wall of the pile cap cylinder body, corresponding to the diameter-variable mechanism; the diameter-variable mechanism comprises a hydraulic cylinder arranged on the base and an outer support positioning block arranged on the outer side surface of a push-against pile body arranged at the end part of a piston rod of the hydraulic cylinder; the inner support mechanism comprises a bearing carrier connected with the pilecap cylinder body, an inner support baffle matching the surface of the pile body is movably clamped on the lower surface of the bearing carrier, the lower part of the bearing carrier is provided witha flange disc, the flange disc is provided with an upper hydraulic lever which is arranged vertically and downwards, the lower end of the upper hydraulic lever is provided with a connecting disc, theconnecting disc is hinged with a connecting rod, and the end part of the connecting rod is hinged with the inner support baffle.