A method for controlling a hydraulic system for a construction machine is disclosed, which can minimize a loss of pressure through expansion of an open area of a confluence spool in the case of singly driving a working device such as a boom. The method for controlling a hydraulic system includes a first step of detecting pilot signals according to operations of operation levers for first and second working devices through first and second pressure sensing devices and transmitting the detected pilot signals to the controller; a second step of determining whether the operation is a single operation of the operation lever for the first working device; a third step of supplying secondary signal pressure by an electro-proportional value to the confluence spool to expand an open area of the confluence spool with respect to the same operation pressure of the operation lever for the first working device and discharging hydraulic fluid discharged from a second hydraulic pump in proportion to the operation amount of the operation lever for the first working device if the operation lever for the first working device is operated to exceed a determined section in the case of singly driving the first working device due to the single operation of the operation lever for the first working device; and a fourth step of supplying the secondary signal pressure by the electro-proportional value to the confluence spool in proportion to the operation amount of the operation lever for the first working device and dividedly supplying the hydraulic fluid discharged from the second hydraulic pump to the first and second working devices in the case of performing a combined driving of the first and second working devices due to simultaneous operations of the operation levers for the first and second working devices.