A swing control system for construction machines, comprising: a hydraulic pump; a swing motor driven by a hydraulic fluid supplied from the hydraulic pump and configured to swing an upper swing structure; a swing control valve installed between the hydraulic pump and the swing motor, and configured to control the flow rate of the hydraulic fluid supplied and returned to the swing motor via a pair of hydraulic lines (13a, 13b); swing manipulation means configured to apply a spool shift signal corresponding to a manipulated variable of an operator to the swing control valve; a shuttle valve installed on a relief line branched off from the hydraulic lines (13a, 13b), and configured to select a higher pressure of the pressures of the hydraulic lines (13a, 13b) and drain a hydraulic fluid of the selected hydraulic line having the higher pressure to the relief line; a relief valve installed on the relief line at the downstream side of the shuttle valve, and configured to control a required flow rate for a soft swing start to drain the hydraulic fluid through a first cutoff valve to a hydraulic tank, when a first cutoff valve is opened during swing start acceleration; characterised by a first cutoff valve installed between the shuttle valve and the relief valve, and configured to selectively control the required flow rate for the soft swing start through the shuttle valve during swing start acceleration; a second cutoff valve installed on a connection line branched off from the pair of hydraulic lines (13a, 13b), and configured to selectively control a required flow rate for a soft swing stop on the connection line during swing stop deceleration; a pair of pressure detecting means (16a, 16b) each configured to sense a pilot signal pressure provided to the swing control valve to shift or switch a spool; and an electric controller electrically connected to the relief valve, the first cutoff valve, the second cutoff valve, and the pressure detecting means (16a, 16b), the electric controller being configured to apply a transfer function to a pressure difference between the pair of pressure detecting means (16a, 16b) to smoothen the pressure difference, to determine whether the upper swing structure is in the soft swing start or stop based on the sign of the derivative function of the smoothened pressure difference, and to selectively output valve control signals for controlling the valves during the swing start acceleration and the swing stop deceleration. a hydraulic pump; a swing motor driven by a hydraulic fluid supplied from the hydraulic pump and configured to swing an upper swing structure; a swing control valve installed between the hydraulic pump and the swing motor, and configured to control the flow rate of the hydraulic fluid supplied and returned to the swing motor via a pair of hydraulic lines (13a, 13b); swing manipulation means configured to apply a spool shift signal corresponding to a manipulated variable of an operator to the swing control valve; a shuttle valve installed on a relief line branched off from the hydraulic lines (13a, 13b), and configured to select a higher pressure of the pressures of the hydraulic lines (13a, 13b) and drain a hydraulic fluid of the selected hydraulic line having the higher pressure to the relief line; a relief valve installed on the relief line at the downstream side of the shuttle valve, and configured to control a required flow rate for a soft swing start to drain the hydraulic fluid through a first cutoff valve to a hydraulic tank, when a first cutoff valve is opened during swing start acceleration; characterised by a first cutoff valve installed between the shuttle valve and the relief valve, and configured to selectively control the required flow rate for the soft swing start through the shuttle valve during swing start acceleration; a second cutoff valve installed on a connection line branched off from the pair of hydraulic lines (13a, 13b), and configured to selectively control a required flow rate for a soft swing stop on the connection line during swing stop deceleration; a pair of pressure detecting means (16a, 16b) each configured to sense a pilot signal pressure provided to the swing control valve to shift or switch a spool; and an electric controller electrically connected to the relief valve, the first cutoff valve, the second cutoff valve, and the pressure detecting means (16a, 16b), the electric controller being configured to apply a transfer function to a pressure difference between the pair of pressure detecting means (16a, 16b) to smoothen the pressure difference, to determine whether the upper swing structure is in the soft swing start or stop based on the sign of the derivative function of the smoothened pressure difference, and to selectively output valve control signals for controlling the valves during the swing start acceleration and the swing stop deceleration.