A winding device for a cordless roller blind, and adapted to be mounted to a supporting structure opposite to a securing seat to cooperate with the securing seat for securing therebetween a blind unit, which includes a pole and a blind wound around the pole, said winding device comprising: a mounting seat adapted to be mounted on the supporting structure; and a winding unit including a casing and a winding set that is mounted to said casing and that is adapted for rolling the pole to bring the blind to fold upwardly and unfold downwardly relative to the securing seat and said mounting seat; characterized by a connecting mechanism for connecting removably said winding unit to said mounting seat in such a manner that said winding unit is upright when connected to said mounting seat, said connecting mechanism including either a first connecting set that is disposed on said casing and at least two second connecting sets (52, 52', 52", 52"') that are disposed on said mounting seat, or a first connecting seat that is disposed on said mounting seat and at least two second connecting sets (52, 52', 52", 52'") that are disposed on said casing, said first connecting set being in each alternative removably engageable with one of said second connecting sets (52, 52', 52", 52'") so as to connect removably said winding unit to said mounting seat wherein said first connecting set includes two first connecting elements (510a, 510b) that are spaced apart from each other, each of said second connecting sets (52, 52', 52", 52'") including two second connecting elements (520, 520', 520", 520"') that are spaced apart from each other; wherein said first connecting elements are one of male connecting elements and female connecting elements, and said second connecting elements (520, 520', 520", 520'") are the other of the male connecting elements and the female connecting elements; wherein one of said female connecting elements is in the form of a hole, and one of said male connecting elements is in the form of a protrusion, said hole having a large dimension portion and a small dimension portion that is connected to said large dimension portion and that is smaller in dimension than said large dimension portion, said protrusion having a head portion and a neck portion that is connected to said head portion and that is smaller in dimension than said head portion; and wherein said protrusion is extendible into said large dimension portion of said hole and is restricted from moving out of said hole by said head portion when said protrusion is moved from having said neck portion extending through said large dimension portion to having said neck portion extending through said small dimension portion by way of rotation between the winding unit and the mounting seat. a mounting seat adapted to be mounted on the supporting structure; and a winding unit including a casing and a winding set that is mounted to said casing and that is adapted for rolling the pole to bring the blind to fold upwardly and unfold downwardly relative to the securing seat and said mounting seat; characterized by a connecting mechanism for connecting removably said winding unit to said mounting seat in such a manner that said winding unit is upright when connected to said mounting seat, said connecting mechanism including either a first connecting set that is disposed on said casing and at least two second connecting sets (52, 52', 52", 52"') that are disposed on said mounting seat, or a first connecting seat that is disposed on said mounting seat and at least two second connecting sets (52, 52', 52", 52'") that are disposed on said casing, said first connecting set being in each alternative removably engageable with one of said second connecting sets (52, 52', 52", 52'") so as to connect removably said winding unit to said mounting seat