A control system for construction machinery, comprising: a first hydraulic pump; first and second actuators (10, 20) connected to the first hydraulic pump through first and second hydraulic lines (210, 220) respectively and operable by a working oil discharged from the first hydraulic pump; first and second control valves (310, 320) installed in the first and second hydraulic lines (210. 220) respectively and configured to control operations of the first and second actuators (10, 20); first spool displacement adjusting valves supplying a signal indicative of pilot pressure to a spool of the first control valve in proportion to an inputted control signal to control a displacement amount of the spool of the first control valve; second spool displacement adjusting valves supplying a signal indicative of pilot pressure to a spool of the second control valve in proportion to an inputted control signal to control a displacement amount of the spool of the second control valve; and a controller configured to output the control signal to the first and second spool displacement adjusting valves corresponding to a manipulation signal of an operator, and configured to adjust a spool displacement amount of the second control valve according to an amount of the working oil to be supplied to the second actuator when the second actuator performs a single or multiple operation by self-load, characterized in that the first actuator includes a boom cylinder and the second actuator includes an arm cylinder, when only an arm joystick is manipulated such that the arm cylinder performs an arm crowd operation by self-load, and when the arm joystick and a boom joystick are manipulated at the same time such that the arm cylinder performs the arm crowd operation by self-load and the boom cylinder performs a boom up operation, the controller outputs the control signal to a meter-out adjusting valve of the second spool displacement adjusting valves to perform a meter-out opening area control of the second control valve such that a meter-out opening area of the second control valve is controlled in proportion to an amount of the working oil to be supplied to the arm cylinder, wherein, in case that only the arm joystick is manipulated such that the arm cylinder performs the arm crowd operation, the meter-out opening area of the second control valve is controlled to be smaller than the meter-out opening area of the second control valve, in case that the arm joystick and the boom joystick are manipulated at the same time such that the arm cylinder performs the arm crowd operation and the boom cylinder performs the boom up operation. a first hydraulic pump; first and second actuators (10, 20) connected to the first hydraulic pump through first and second hydraulic lines (210, 220) respectively and operable by a working oil discharged from the first hydraulic pump; first and second control valves (310, 320) installed in the first and second hydraulic lines (210. 220) respectively and configured to control operations of the first and second actuators (10, 20); first spool displacement adjusting valves supplying a signal indicative of pilot pressure to a spool of the first control valve in proportion to an inputted control signal to control a displacement amount of the spool of the first control valve; second spool displacement adjusting valves supplying a signal indicative of pilot pressure to a spool of the second control valve in proportion to an inputted control signal to control a displacement amount of the spool of the second control valve; and a controller configured to output the control signal to the first and second spool displacement adjusting valves corresponding to a manipulation signal of an operator, and configured to adjust a spool displacement amount of the second control valve according to an amount of the working oil to be supplied to the second actuator when the second actuator performs a single or multiple operation by self-load,