The invention belongs to the technical field of metallurgical machinery and discloses a three-supporting surface prestress blocking seat mechanism of a converter. The three-supporting surface prestress blocking seat mechanism of the converter mainly comprises a supporting ring, a converter shell, supporting ring blocking seats and a converter shell seat assembly, wherein the supporting ring blocking seats are welded on the supporting ring, the converter shell seat is welded on the converter shell, the upper parts of the supporting ring blocking seats of both sides are connected by a pretensionpull rod and a linking member and are pretensioned inwards, the converter shell seat is clamped between the supporting ring blocking seats, the converter shell seat and the supporting ring blocking seats are provided with two lateral contact supporting surfaces, a longitudinal contact cushion block is additionally arranged between the supporting ring and the converter shell seat, and a longitudinal contact supporting surface is arranged between the supporting ring and the converter shell seat. The three-supporting surface prestress blocking seat mechanism effectively restricts a lateral gap and longitudinal elastic displacement of a converter body in the supporting ring, increases the operation stability of the converter, prolongs the service life, improves the reliability of the converter and is a novel technology with a popularization value on the converter.