A hydraulic system for construction machinery is capable of suppressing irregularity in the operation of the front structure not intended by the operator, and includes a regeneration motor adapted to be driven by return fluid from a bottom port of a boom cylinder; a regeneration electric motor converting rotary power into electric energy; return fluid control valves controlling the flow of the return fluid according to a pilot pressure related to an operation of a boom control lever; and a malfunction prevention device adapted to be activated by pressure fluctuation caused by malfunction of a solenoid valve for the return fluid control valve that interrupts the delivery of a pilot secondary pressure from the solenoid valve to the corresponding return fluid control valve.