A hydraulic system for a construction machine, comprising: a pump/motor configured to serve as both a hydraulic pump driven by an engine and discharging working oil and a motor generating rotational force by the working oil; an actuator operated by receiving hydraulic pressure from the pump/motor and provided with first and second ports (170a and 170b) through which the hydraulic pressure flows in and out; first and second hydraulic pressure lines (1La and 1Lb) configured to connect the pump/motor and the actuator, in which a first pressure (Pa) and a second pressure (Pb) are formed respectively; an accumulator configured to store or discharge the working oil through the first and second hydraulic pressure lines (1La and 1Lb) and first and second bypass lines (1411 and 1412); first and second check valve units (610 and 620) provided on the first and second bypass lines (1411 and 1412) respectively and configured to allow the working oil to move only to the first and second hydraulic pressure lines (1La and 1Lb); and a control valve unit, of which both pressure receiving portions are connected with the first and second hydraulic pressure lines (1La and 1Lb), and switched so that a hydraulic pressure line having lower pressure between the first and second hydraulic pressure lines communicates with the accumulator, characterized in that the control valve unit comprises an internal flow path comprising a second position connecting the second hydraulic pressure line and the accumulator, a third position connecting the first hydraulic pressure line and the accumulator, and a first position blocking hydraulic pressure from flowing to any one side, and has a spool structure, in which the first pressure (Pa) and the second pressure (Pb) of the first and second hydraulic pressure lines (1La and 1Lb) are applied to both pressure receiving portions. a pump/motor configured to serve as both a hydraulic pump driven by an engine and discharging working oil and a motor generating rotational force by the working oil; an actuator operated by receiving hydraulic pressure from the pump/motor and provided with first and second ports (170a and 170b) through which the hydraulic pressure flows in and out; first and second hydraulic pressure lines (1La and 1Lb) configured to connect the pump/motor and the actuator, in which a first pressure (Pa) and a second pressure (Pb) are formed respectively; an accumulator configured to store or discharge the working oil through the first and second hydraulic pressure lines (1La and 1Lb) and first and second bypass lines (1411 and 1412); first and second check valve units (610 and 620) provided on the first and second bypass lines (1411 and 1412) respectively and configured to allow the working oil to move only to the first and second hydraulic pressure lines (1La and 1Lb); and a control valve unit, of which both pressure receiving portions are connected with the first and second hydraulic pressure lines (1La and 1Lb), and switched so that a hydraulic pressure line having lower pressure between the first and second hydraulic pressure lines communicates with the accumulator, characterized in that the control valve unit comprises an internal flow path comprising a second position connecting the second hydraulic pressure line and the accumulator, a third position connecting the first hydraulic pressure line and the accumulator, and a first position blocking hydraulic pressure from flowing to any one side, and has a spool structure, in which the first pressure (Pa) and the second pressure (Pb) of the first and second hydraulic pressure lines (1La and 1Lb) are applied to both pressure receiving portions.