A method of controlling a flow rate of a variable displacement hydraulic pump for a construction machine which comprises: a variable displacement hydraulic pump, a hydraulic actuator connected to the hydraulic pump, a manipulation lever configured to output a manipulation signal that is in proportion to a manipulation amount of the manipulation lever by a user, a control valve configured to control a start, a stop, and a direction change of the hydraulic actuator when it is shifted in response to the manipulation signal outputted from the manipulation lever, a manipulation amount detection means configured to detect the manipulation amount of the manipulation lever, a discharge pressure detection sensor configured to detect a pressure of a hydraulic fluid discharged from the hydraulic pump, and a controller configured to control a discharge flow rate of the hydraulic pump in response to detection signals outputted from the manipulation amount detection means and the discharge pressure detection sensor, the method comprising: a first step of calculating a discharge flow rate of the hydraulic fluid, which is required by the hydraulic pump according to the manipulation amount of the manipulation lever by a user; a second step of calculating an allowable discharge flow rate (Q available) within a range that does not exceed a preset specific horsepower or torque of the hydraulic pump, relative to the discharge pressure of the hydraulic fluid detected by the discharge pressure detection sensor; and a third step of controlling a discharge flow rate of the hydraulic pump in proportion to the manipulation amount of the manipulation lever within a range of the allowable discharge flow rate value calculated in the second step, characterized in that the discharge flow rate of the hydraulic pump is calculated by calculating the discharge flow rate required by the hydraulic pump according to the manipulation amount of the manipulation lever as a percentage under the no-load condition and multiplying the allowable discharge flow rate of the hydraulic pump relative to the preset discharge pressure by the calculated percentage. a first step of calculating a discharge flow rate of the hydraulic fluid, which is required by the hydraulic pump according to the manipulation amount of the manipulation lever by a user; a second step of calculating an allowable discharge flow rate (Q available) within a range that does not exceed a preset specific horsepower or torque of the hydraulic pump, relative to the discharge pressure of the hydraulic fluid detected by the discharge pressure detection sensor; and a third step of controlling a discharge flow rate of the hydraulic pump in proportion to the manipulation amount of the manipulation lever within a range of the allowable discharge flow rate value calculated in the second step, characterized in that the discharge flow rate of the hydraulic pump is calculated by calculating the discharge flow rate required by the hydraulic pump according to the manipulation amount of the manipulation lever as a percentage under the no-load condition and multiplying the allowable discharge flow rate of the hydraulic pump relative to the preset discharge pressure by the calculated percentage.