Disclosed is a hydraulic control system of a construction machine capable of achieving a reduction in meter-out pressure loss in accordance with variation of a negative load acting on a hydraulic actuator and capable of preventing deterioration in operability even when an abnormal condition arises in a pressure sensor detecting the magnitude of a negative load. There are provided meter-out flow lines through which a hydraulic fluid discharged from a hydraulic actuator flows; variable restrictors provided in the meter-out flow lines; a load sensor detecting the magnitude of a negative load which is a load applied to the hydraulic actuator by an external force and which is a load in the same direction as the operating direction of the hydraulic actuator; and a control device which, when the load abnormality sensor does not detect any abnormality in the load sensor, reduces the sum total of the opening areas of the variable restrictors in accordance with an increase in the magnitude of the negative load detected by the load sensor and the operation amount detected by the operation amount sensor and which, when the load abnormality sensor detects abnormality in the load sensor, reduces the sum total of the opening areas of the variable restrictors to a predetermined value in accordance with the operation amount detected by the operation amount sensor.