A control system for construction machinery includes first and second hydraulic pumps connected to an engine, a swing control valve and a first boom control valve installed in a first center bypass line connected to the first hydraulic pump, a second boom control valve installed in a second center bypass line connected to the second hydraulic pump, a shutoff valve installed in a control line through which a boom raising pilot signal pressure is supplied to the first boom control valve and configured to selectively open and close the control line, and a control unit configured to electronically control the shutoff valve according to a manipulation signal, wherein the control unit closes the shutoff valve such that the first boom control valve is shifted to a neutral position when a swing operation priority mode is selected.