The invention relates to a high-power hydraulic mechanical gearbox and a control method thereof. The high-power hydraulic mechanical gearbox comprises a hydraulic torque converter, a hydraulic retarder, a shifting clutch and a mechanical transmission device which are arranged from front to back sequentially; the hydraulic torque converter is provided with a lock-up clutch, the input end of the hydraulic torque converter is connected with an engine, and the output end of the hydraulic torque converter is connected with the input end of the shifting clutch through a transmission shaft; a rotor impeller of the hydraulic retarder is connected with the transmission shaft through a retarding gear; the shaft output end of the shifting clutch is connected with the shaft input end of the mechanicaltransmission device; and the mechanical transmission device is provided with a first power output shaft. The high-power hydraulic mechanical gearbox has the advantages of compact structure, large output torque, high reliability and good stability. The control method has the advantages of simple process, high degree of automation, safety and reliability.