The present disclosure relates to a hydraulic system for construction machinery, which is applied to construction machinery provided with a steering unit for controlling steering of equipment, and a main control valve for controlling operating of a work tool, and includes: a main pump for discharging working fluid supplied to the steering unit and the main control valve; a load sensing pressure sensor for sensing a load sensing pressure of the steering unit; a priority valve installed in a supply passage for the working fluid discharged from the main pump, for dividing and supplying the working fluid discharged from the main pump to the steering unit and the main control valve, to correspond to the load sensing pressure; an auxiliary pump for supplementing working fluid to the supply passage supplying the working fluid divided by the priority valve to the steering unit; an electric motor for operating the auxiliary pump; and a controller for operating the electric motor when the load sensing pressure is greater than a reference load sensing pressure, and additionally supplying working fluid discharged from the auxiliary pump the steering unit.