Disclosed is a hydraulic pump for construction machinery for controlling to inhibit one-way driving when compound-operating two-way driving and work devices, such as a boom, to enhance operation efficiency. The hydraulic system for the construction machinery of the present invention provides the hydraulic system comprising: an operation device for driving and an operation lever for the work devices; a left driving motor which connects to a first hydraulic pump; a first control valve which is installed on the discharge flow path of the first hydraulic pump; a right driving motor which is connected to a second hydraulic pump; a hydraulic actuator which is connected to the first hydraulic pump and the second hydraulic pump; a second control valve which is installed on the discharge flow path of the first hydraulic pump or the second hydraulic pump; a third control valve which is installed on a flow path that branches from the discharge flow path of the second flow path; a first bypass valve which is connected to the upstream portion of the discharge flow path of the first hydraulic pump; a second bypass valve which is connected to the upstream portion of the discharge flow path of the second hydraulic pump; a confluence valve which is installed on a flow path which connects in parallel the discharge paths of the first and second hydraulic pumps; and a controller for controlling the opening of the first and second bypass valves and the confluence valve according to an operation signal that is input from the operation device for driving and the operation lever for the work device.