A hydraulic pump controlling apparatus of a construction machine, comprising: a hydraulic pump in which a swash plate angle is controlled to control a discharge flow; an auxiliary pump; a control valve controlling a flowing direction of a fluid discharged from the hydraulic pump and selectively supplying the fluid to an actuator; an orifice and a relief valve connected between a center bypass line of the control valve and a tank to be parallel to each other; signal pressure selecting units (20, 120, and 220) receiving a fluid passing through the center bypass line of the control valve and a fluid discharged from the auxiliary pump, and selecting the pressure of any one of the fluids as a signal pressure; and a regulator receiving the signal pressure selected from the signal pressure selecting units (20, 120, and 220) to control the swash plate angle of the hydraulic pump, wherein the signal pressure selecting unit selects the pressure of the fluid discharged from the auxiliary pump as the signal pressure and transfers the selected pressure to the regulator when the construction machine is in an idle state, characterized in that the signal pressure selecting unit includes: a valve unit changed between a first position where the pressure of the auxiliary pump is transferred to the regulator and a working fluid of the center bypass line is discharged to the tank and a second position where the pressure of the center bypass line is transferred to the regulator; and a controller changing the valve unit to the first position when a work signal (P i) is not inputted and changing the valve unit to the second position when the work signal (P i) is inputted. a hydraulic pump in which a swash plate angle is controlled to control a discharge flow; an auxiliary pump; a control valve controlling a flowing direction of a fluid discharged from the hydraulic pump and selectively supplying the fluid to an actuator; an orifice and a relief valve connected between a center bypass line of the control valve and a tank to be parallel to each other; signal pressure selecting units (20, 120, and 220) receiving a fluid passing through the center bypass line of the control valve and a fluid discharged from the auxiliary pump, and selecting the pressure of any one of the fluids as a signal pressure; and a regulator receiving the signal pressure selected from the signal pressure selecting units (20, 120, and 220) to control the swash plate angle of the hydraulic pump, wherein the signal pressure selecting unit selects the pressure of the fluid discharged from the auxiliary pump as the signal pressure and transfers the selected pressure to the regulator when the construction machine is in an idle state, a valve unit changed between a first position where the pressure of the auxiliary pump is transferred to the regulator and a working fluid of the center bypass line is discharged to the tank and a second position where the pressure of the center bypass line is transferred to the regulator; and a controller changing the valve unit to the first position when a work signal (P i) is not inputted and changing the valve unit to the second position when the work signal (P i) is inputted.