Disclosed is a system for controlling a hydraulic pump in a construction machine which can reduce the discharge flow of the hydraulic pump in proportion to the load pressure of the hydraulic pump to enhance operability by the driver. The present invention provides the system for controlling the hydraulic pump in the construction machine, which is characterized by detecting the discharge pressure of the hydraulic pump and the operation activity of an operation lever, and comparing the detected discharge pressure of the hydraulic pump and a predetermined pressure value, and when the detected discharge pressure exceeds the predetermined pressure value, the maximum dischargeable flow of the hydraulic pump is reduced in proportion to the difference between the detected pressure and the predetermined pressure value.