A hydraulic control system for a combined boom and swing operation of a construction machine, which includes first and second variable displacement hydraulic pumps (1, 2), an actuator for a swing device and a work apparatus, which is connected to the first hydraulic pump, an actuator for a work apparatus connected to the second hydraulic pump, and a first flow rate control valve configured to control a flow rate of a hydraulic fluid supplied respectively to the actuators for the swing device and the work apparatus from the first and second hydraulic pumps (1, 2), the hydraulic control system comprising: a work mode selection switch; a proportional pressure control valve mounted in a pilot fluid path that discharges a pilot signal pressure according to a manipulation amount of a boom manipulation lever when the boom manipulation lever is manipulated to perform a boom-up operation, the proportional pressure control valve being configured to output a control signal pressure that is in proportion to the manipulation of the work mode selection switch; a second variable flow rate control valve mounted in a flow path between the first hydraulic pump and a swing device spool for the swing device, the second variable flow rate control valve having an opening area variably adjusted in proportion to the control signal pressure of the proportional pressure control valve; a boom confluence spool; and a controller; characterized in that the proportional pressure control valve is an electronic proportional pressure control valve, the work mode selection switch is configured to select a preset work mode according to working conditions, the controller is configured to set and store various kinds of work modes according to working conditions to control a flow rate of the hydraulic fluid supplied to the swing device spool and the boom confluence spool according to a work mode selected when the work mode selection switch is manipulated, in the case where an operator selects a certain work mode according to the working conditions during a combined operation in which boom-up and swing manipulations are carried out, the flow rate of the hydraulic fluid supplied to the swing device spool and the boom confluence spool is variably controlled according to the selected work mode to adjust the driving speeds of the boom and the swing device, a part of the pilot signal pressure passing by the boom manipulation lever is supplied to a boom spool to shift the boom spool according to the boom-up manipulation of the boom manipulation lever so that the hydraulic fluid discharged from the second hydraulic pump is supplied to a boom cylinder after passing through the boom spool to perform the boom-up operation, and another part of the pilot signal pressure passing by the boom manipulation lever is supplied to the electronic proportional pressure control valve mounted in the pilot flow path. a work mode selection switch; a proportional pressure control valve mounted in a pilot fluid path that discharges a pilot signal pressure according to a manipulation amount of a boom manipulation lever when the boom manipulation lever is manipulated to perform a boom-up operation, the proportional pressure control valve being configured to output a control signal pressure that is in proportion to the manipulation of the work mode selection switch; a second variable flow rate control valve mounted in a flow path between the first hydraulic pump and a swing device spool for the swing device, the second variable flow rate control valve having an opening area variably adjusted in proportion to the control signal pressure of the proportional pressure control valve; a boom confluence spool; and a controller; characterized in that the proportional pressure control valve is an electronic proportional pressure control valve, the work mode selection switch is configured to select a preset work mode according to working conditions, the controller is configured to set and store various kinds of work modes according to working conditions to control a flow rate of the hydraulic fluid supplied to the swing device spool and the boom confluence spool according to a work mode selected when the work mode selection switch is manipulated, in the case where an operator selects a certain work mode according to the working conditions during a combined operation in which boom-up and swing manipulations are carried out, the flow rate of the hydraulic fluid supplied to the swing device spool and the boom confluence spool is variably controlled according to the selected work mode to adjust the driving speeds of the boom and the swing device, a part of the pilot signal pressure passing by the boom manipulation lever is supplied to a boom spool to shift the boom spool according to the boom-up manipulation of the boom manipulation lever so that the hydraulic fluid discharged from the second hydraulic pump is supplied to a boom cylinder after passing through the boom spool to perform the boom-up operation, and another part of the pilot signal pressure passing by the boom manipulation lever is supplied to the electronic proportional pressure control valve mounted in the pilot flow path.