The invention provides a hydraulic tensioning system for rope length control of a hydraulic winch, and belongs to the field of hydraulic winches. The hydraulic tensioning system for rope length control of the hydraulic winch comprises a hydraulic circuit and an electric control circuit. The hydraulic circuit comprises an oil inlet pipeline, an oil return pipeline, an electric proportional reversing valve, a shuttle valve, a counterbalance valve, a hydraulic-control pressure reduction reversing valve, a hydraulic motor, a pilot overflow valve, a one-way valve, a normally-closed brake, a two-position and two-way electromagnetic valve and a direct-acting overflow valve. The electric control circuit comprises two pressure sensors and a rotary encoder, wherein the two pressure sensors are arranged at an oil inlet A and an oil inlet B of the hydraulic motor and used for monitoring the pressures of the two ends of the winch correspondingly, and the rotary encoder is arranged at the output end of the winch so as to be used for monitoring rope length changes. According to the hydraulic tensioning system for rope length control of the hydraulic winch, the pressure difference of an oil inlet/outlet cavity of the winch is controlled through electric and hydraulic combined control according to the passive pressure principle of the system, so that stable steel wire rope tension is obtained, the posture of a steel wire rope is effectively kept, and operating safety of the winch is guaranteed.