A hydraulic control system for a construction machine, the hydraulic control system comprising: a hydraulic pump group comprising a first hydraulic pump, a second hydraulic pump and a third hydraulic pump; a main control valve comprising: a first spool group configured to control flow rates and flows of pressurized oils to be supplied from the first hydraulic pump and the second hydraulic pump and comprising a spool for a working device, a spool for traveling and a spool for confluence, a second spool group configured to control a flow rate and flow of pressurized oil to be supplied from the third hydraulic pump and comprising a spool for swing, and a straight traveling valve equipped between the first spool group and the second spool group; a first pilot signal line connected to the spool for a working device and configured to provide a supply passage of pilot pressurized oil to be applied so as to switch the spool for a working device; a second pilot signal line connected to the spool for traveling and configured to provide a supply passage of pilot pressurized oil to be applied so as to switch the spool for traveling, and a direction changing valve equipped on the second pilot signal line configured to interconnect the spool for traveling and a tank, connected to the first pilot signal line, and configured to be switched upon switching of the spool for a working device, thereby cutting off flow of pilot pressurized oil returning to the tank. a hydraulic pump group comprising a first hydraulic pump, a second hydraulic pump and a third hydraulic pump; a main control valve comprising: a first spool group configured to control flow rates and flows of pressurized oils to be supplied from the first hydraulic pump and the second hydraulic pump and comprising a spool for a working device, a spool for traveling and a spool for confluence, a second spool group configured to control a flow rate and flow of pressurized oil to be supplied from the third hydraulic pump and comprising a spool for swing, and a straight traveling valve equipped between the first spool group and the second spool group; a first spool group configured to control flow rates and flows of pressurized oils to be supplied from the first hydraulic pump and the second hydraulic pump and comprising a spool for a working device, a spool for traveling and a spool for confluence, a second spool group configured to control a flow rate and flow of pressurized oil to be supplied from the third hydraulic pump and comprising a spool for swing, and a straight traveling valve equipped between the first spool group and the second spool group; a first pilot signal line connected to the spool for a working device and configured to provide a supply passage of pilot pressurized oil to be applied so as to switch the spool for a working device; a second pilot signal line connected to the spool for traveling and configured to provide a supply passage of pilot pressurized oil to be applied so as to switch the spool for traveling, and a direction changing valve equipped on the second pilot signal line configured to interconnect the spool for traveling and a tank, connected to the first pilot signal line, and configured to be switched upon switching of the spool for a working device, thereby cutting off flow of pilot pressurized oil returning to the tank.