The utility model relates to a hydraulic loosening preventing joint for a sliding tapping hole oil cylinder of a converter. The loosening preventing joint is characterized by comprising a concave surface joint body (1), a first flange (2), high-strength bolts (3), a second flange (4), a high-temperature loosening preventing disc spring (5), a high-temperature pre-tensioning disc spring (6) and a convex surface joint body (7), wherein a guide surface (8) and a sealing spherical surface (9) are respectively oppositely arranged on the concave surface joint body (1) and the convex surface joint body (7); the high-temperature pre-tensioning disc spring (6) is in match with the second flange (4) and mounted on the convex surface joint body (7); the first flange (2) and the second flange (4) are used for integrally connecting the concave surface joint body (1) and the convex surface joint body (7) through the high-strength bolts (3); the high-temperature loosening preventing disc spring (5) is arranged in match with the high-strength bolts (3). The loosening preventing joint is simple in structure, can meet the requirement on use of a joint for the sliding tapping hole oil cylinder of the converter, and can increase the working rate of the converter.