The invention provides a hydraulic drive system for a construction machine, the system allowing automatic changes of the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom lowering operation. The hydraulic drive system includes a controller 42 for controlling a solenoid switch valve 40 such that a pilot oil passage 38b with a pressure reducing valve 39 is selected to set the limit of a boom-lowering spool stroke of a boom directional control valve 31 to a middle position L1 when the rod-side pressure of a hydraulic boom cylinder 20 detected by a pressure sensor 41 is less than a threshold value and such that a pilot oil passage 38a is selected to set the limit of the boom-lowering spool stroke of the boom directional control valve 31 to a maximum stroke position L2 when the rod-side pressure is equal to or greater than the threshold value.
