The invention discloses an online servo large-diameter bar hydraulic follow-up fixed-length shearing mechanism. The online servo large-diameter bar hydraulic follow-up fixed-length shearing mechanism mainly comprises a fixed bracket, a hydraulic shearing machine, a mechanical connecting mechanism and a bearing frame, wherein the fixed bracket mainly comprises a left bracket, a right bracket, a shearing machine guide rail, a guide sleeve, a photoelectric sensor and the like. The hydraulic shearing machine mainly comprises a hydraulic cylinder, an upper cutting knife, a lower cutting knife, a shearing machine roller, a speed measuring support wheel and a bracket. The bearing frame mainly comprises an upper bracket, a lower horizontal rod, a lower bracket, a lower bracket roller, a linear guide rail, a wide guide rail, a baffle rod, a turning arm, a bracket and a fixed-length baffle plate. The mechanical connecting mechanism comprises a servo motor, a gear, a rack, a connecting rod and a touch plate. The online servo large-diameter bar hydraulic follow-up fixed-length shearing mechanism can largely quicken the bar speed to realize high-speed online production, and prevents secondary processing and resource waste to achieve high practical value.