A hydraulic system for construction machinery comprises a regeneration valve unit including a first opening/closing valve installed in a first hydraulic line which connects a head-side chamber of a boom cylinder and a drain tank, to open and close the first hydraulic line, and a second opening/closing valve installed in a second hydraulic line which is branched from the first hydraulic line and is connected to a rod-side chamber of the boom cylinder, to open and close the second hydraulic line, a first check valve installed in the second hydraulic line between the rod-side chamber and the regeneration valve unit and configured to selectively drain a working oil discharged from the rod-side chamber to the drain tank, and a control unit configured to control opening and closing of the first opening/closing valve, the second opening/closing valve and the first check valve according to a control mode.