A heatsink-locking device that is applied to lock a heatsink to a placement surface having an exothermic component, includes a bottom base fixed on the placement surface to dispose and position the heatsink and having an open hole corresponding to the exothermic component and clamping portions at two opposing lateral sides; and a clamping tool for pressing the heatsink and being jointed to the bottom base, the clamping tool including a first board part and a second board part connected to each other with a distance between these two parts being adjustable, a third board part axially connected to the first board part to fix the adjustable distance, a pressing part fixed to the third board part to press the heatsink, and a first joint part and a second joint part disposed at one end of the first and second board parts respectively to be jointed to the clamping portion.