An apparatus for controlling displacement of a spool of a construction machine, which comprises a main pump driven by an engine, actuators driven by working oil discharged from the main pump, a main control valve configured to control driving of the actuator according to a quantity of displacement of a spool installed therein, a joystick configured to adjust the main control valve, and a pilot gear pump configured to discharge working oil for driving the main control valve, the apparatus further comprises: a control unlit configured to generate a signal for the quantity of displacement of the spool according to a quantity of operation of the joystick and a current pressure of the main pump; a spool displacement adjusting unit configured to adjust the quantity of displacement of the spool according to the signal generated by the control unit; and a hydraulic line configured to supply working oil discharged from the pilot gear pump to the spool displacement adjusting unit, wherein the control unit includes: a joystick operation quantity determining unit configured to determine the quantity of operation of the Joystick; a main pump pressure comparing unit configured to compare the current pressure of the main pump with a predetermined operating pressure; characterised in that the control unit further includes: a signal generating unit configured to generate a signal for controlling the spool to have a value of displacement quantity smaller than a maximum permissible displacement quantity of the spool as recognized to be in a regeneration mode when the current pressure of the main pump is less than the predetermined operating pressure as a result of the determination of the main pump pressure comparing unit, and to generate a signal for changing the spool to have a value of the maximum permissible displacement quantity of the spool as determined to be in an operating mode when the pressure of the main pump is greater than or equal to the predetermined operating pressure. a control unlit configured to generate a signal for the quantity of displacement of the spool according to a quantity of operation of the joystick and a current pressure of the main pump; a spool displacement adjusting unit configured to adjust the quantity of displacement of the spool according to the signal generated by the control unit; and a hydraulic line configured to supply working oil discharged from the pilot gear pump to the spool displacement adjusting unit, wherein the control unit includes: a joystick operation quantity determining unit configured to determine the quantity of operation of the Joystick; a main pump pressure comparing unit configured to compare the current pressure of the main pump with a predetermined operating pressure; characterised in that the control unit further includes: a signal generating unit configured to generate a signal for controlling the spool to have a value of displacement quantity smaller than a maximum permissible displacement quantity of the spool as recognized to be in a regeneration mode when the current pressure of the main pump is less than the predetermined operating pressure as a result of the determination of the main pump pressure comparing unit, and to generate a signal for changing the spool to have a value of the maximum permissible displacement quantity of the spool as determined to be in an operating mode when the pressure of the main pump is greater than or equal to the predetermined operating pressure. a joystick operation quantity determining unit configured to determine the quantity of operation of the Joystick; a main pump pressure comparing unit configured to compare the current pressure of the main pump with a predetermined operating pressure; characterised in that the control unit further includes: a signal generating unit configured to generate a signal for controlling the spool to have a value of displacement quantity smaller than a maximum permissible displacement quantity of the spool as recognized to be in a regeneration mode when the current pressure of the main pump is less than the predetermined operating pressure as a result of the determination of the main pump pressure comparing unit, and to generate a signal for changing the spool to have a value of the maximum permissible displacement quantity of the spool as determined to be in an operating mode when the pressure of the main pump is greater than or equal to the predetermined operating pressure.