A hydraulic control system for a construction machine, the hydraulic control system comprising: a hydraulic pump group comprising a first hydraulic pump (P1), a second hydraulic pump (P2) and a third hydraulic pump (P3) ; a main control valve (MCV) comprising: a first spool group configured to control flow rates and flows of pressurized oils to be supplied from the first hydraulic pump (P1) and the second hydraulic pump (P2) 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 (P3) 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 (P1), a second hydraulic pump (P2) and a third hydraulic pump (P3) ; a main control valve (MCV) comprising: a first spool group configured to control flow rates and flows of pressurized oils to be supplied from the first hydraulic pump (P1) and the second hydraulic pump (P2) 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 (P3) 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 (P1) and the second hydraulic pump (P2) 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 (P3) 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 . <EOS>