A method for controlling a hydraulic pump control system for a construction machine, wherein the system comprising a variable displacement hydraulic pump, at least one hydraulic actuator connected to the hydraulic pump, a spool configured to control hydraulic fluid supplied to the actuator when the spool is shifted by a signal pressure that is supplied to the spool in proportion to a manipulation amount of a manipulation lever, a discharge pressure detection sensor installed in a discharge flow path of the hydraulic pump and configured to detect a discharge pressure of the hydraulic pump, a signal pressure detection sensor configured to detect the signal pressure based on the manipulation amount of the manipulation lever, and a control unit configured to control a discharge flow rate of the hydraulic pump in response to the detection signals from the detection sensors (7, 8), the method comprising detecting the discharge pressure of the hydraulic pump by the discharge pressure detection sensor and the manipulation amount of the manipulation lever by the signal pressure detection sensor, respectively; calculating a required flow rate of the hydraulic pump based on the manipulation amount of the manipulation lever (4; S200); characterized in that the method further comprises: comparing the sizes of the detected discharge pressure of the hydraulic pump and a preset pressure that is obtained by subtracting a given value from a set pressure of a main relief valve; proportionally reducing, if the detected discharge pressure of the hydraulic pump is more than the preset pressure, a maximum dischargeable flow rate of the hydraulic pump based on a difference between the detected discharge pressure and the preset pressure; comparing a currently required discharge flow rate of the hydraulic pump with the reduced maximum dischargeable flow rate (Q') of the hydraulic pump (2; S500); and controlling the discharge flow rate of the hydraulic pump to be the maximum dischargeable flow rate if the currently required discharge flow rate of the hydraulic pump exceeds the maximum dischargeable flow rate (Q, S600A), while controlling the discharge flow rate of the hydraulic pump to be the required flow rate of the hydraulic pump, which is calculated based on the manipulation amount, if the currently required discharge flow rate of the hydraulic pump is less than the maximum dischargeable flow rate (Q; S600B), wherein the control unit controls, if the detected discharge pressure is more than the preset pressure even in the case where the required discharge flow rate of the hydraulic pump does not reach the maximum dischargeable flow rate, the required discharge flow rate of the hydraulic pump to be proportionally reduced based on a difference between the detected discharge pressure and the preset pressure in such a manner that the degree of reduction is proportionally controlled based on the size of the discharge flow rate so that the reduction rate of the discharge flow rate is controlled to be equal to or to approximate the size of the required discharge flow rate of the hydraulic pump. comparing the sizes of the detected discharge pressure of the hydraulic pump and a preset pressure that is obtained by subtracting a given value from a set pressure of a main relief valve; proportionally reducing, if the detected discharge pressure of the hydraulic pump is more than the preset pressure, a maximum dischargeable flow rate of the hydraulic pump based on a difference between the detected discharge pressure and the preset pressure; comparing a currently required discharge flow rate of the hydraulic pump with the reduced maximum dischargeable flow rate (Q') of the hydraulic pump (2; S500); and controlling the discharge flow rate of the hydraulic pump to be the maximum dischargeable flow rate if the currently required discharge flow rate of the hydraulic pump exceeds the maximum dischargeable flow rate (Q, S600A), while controlling the discharge flow rate of the hydraulic pump to be the required flow rate of the hydraulic pump, which is calculated based on the manipulation amount, if the currently required discharge flow rate of the hydraulic pump is less than the maximum dischargeable flow rate (Q; S600B), wherein the control unit controls, if the detected discharge pressure is more than the preset pressure even in the case where the required discharge flow rate of the hydraulic pump does not reach the maximum dischargeable flow rate, the required discharge flow rate of the hydraulic pump to be proportionally reduced based on a difference between the detected discharge pressure and the preset pressure in such a manner that the degree of reduction is proportionally controlled based on the size of the discharge flow rate so that the reduction rate of the discharge flow rate is controlled to be equal to or to approximate the size of the required discharge flow rate of the hydraulic pump.