A flow control valve for a construction machine, the flow control valve controlling hydraulic fluid supplied from a hydraulic pump to a hydraulic actuator, the flow control valve comprising: a boom valve block connected to the hydraulic pump; a boom spool slidably installed in the boom valve block and shifted to control hydraulic fluid supplied from the hydraulic pump to a boom cylinder in response to pilot signal pressure supplied to perform a boom-up or boom-down operation; and a boom-down flow control means for returning a part of hydraulic fluid returning from a large chamber of the boom cylinder to a hydraulic tank, through the boom spool only at a predetermined pilot pressure or higher when the boom spool is shifted to perform the boom-down operation, and for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder, directly to the hydraulic tank through the boom spool and allowing a part of the hydraulic fluid returning from the large chamber of the boom cylinder to join a small chamber of the boom cylinder as a regeneration flow when the boom spool moves over a predetermined section, wherein the boom-down flow control means includes a boom booster poppet mounted on the boom valve block in a path for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder to the hydraulic tank through a first spool notch of the boom spool to be opened only at the predetermined pilot pressure or higher during the boom-down operation, and characterized in that the boom-down flow control means includes a second spool notch provided to be spaced apart from the first spool notch of the boom spool and to communicate with the path for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder to the hydraulic tank so as to return the part of the hydraulic fluid directly to the hydraulic tank without passing through the boom booster poppet when the boom spool moves over the predetermined section during the boom-down operation. a boom valve block connected to the hydraulic pump; a boom spool slidably installed in the boom valve block and shifted to control hydraulic fluid supplied from the hydraulic pump to a boom cylinder in response to pilot signal pressure supplied to perform a boom-up or boom-down operation; and a boom-down flow control means for returning a part of hydraulic fluid returning from a large chamber of the boom cylinder to a hydraulic tank, through the boom spool only at a predetermined pilot pressure or higher when the boom spool is shifted to perform the boom-down operation, and for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder, directly to the hydraulic tank through the boom spool and allowing a part of the hydraulic fluid returning from the large chamber of the boom cylinder to join a small chamber of the boom cylinder as a regeneration flow when the boom spool moves over a predetermined section, wherein the boom-down flow control means includes a boom booster poppet mounted on the boom valve block in a path for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder to the hydraulic tank through a first spool notch of the boom spool to be opened only at the predetermined pilot pressure or higher during the boom-down operation, and characterized in that the boom-down flow control means includes a second spool notch provided to be spaced apart from the first spool notch of the boom spool and to communicate with the path for returning a part of the hydraulic fluid returning from the large chamber of the boom cylinder to the hydraulic tank so as to return the part of the hydraulic fluid directly to the hydraulic tank without passing through the boom booster poppet when the boom spool moves over the predetermined section during the boom-down operation.