The present invention relates to a variable fastener for fixing a curtain wall, enabling a vertical member of a curtain wall unit to be coupled to an end portion of a building slab (S), wherein the variable fastener comprises: a first bracket fixed to the end portion of the slab (S); and a second bracket, one side of which is coupled to the vertical member of the curtain wall unit, and the other side of which is coupled to the first bracket, so as to transfer the load of the curtain wall unit to the slab (S). The first bracket includes a planar base portion fixed to the end portion of the slab (S), and a support portion formed on the front end of the base portion so as to be higher than the upper surface of the base portion. The second bracket includes: a planar fixing portion fixed to the vertical member of the curtain wall unit; a height-adjusting bolt coupled and fixed perpendicularly at one side end of the fixing portion that is in the direction of the first bracket; a catching portion, one side of which is coupled so as to be horizontally pivotable and vertically slidable on the height-adjusting bolt, and the other side of which is mounted on an upper part of the support portion; and a height-adjusting nut fastened to the height-adjusting bolt, for adjusting the height of the catching portion.