Abstract: A folding device for a cordless blind includes a string spool and a force-return mechanism. The force-return mechanism includes a transmission shaft coupled to the string spool to turn therewith. A smaller bevel gear is connected to the transmission shaft and meshes with a larger bevel gear rotatable about a vertical axis. A first bobbin is coupled to the larger bevel gear to turn therewith. A spiral spring has two ends respectively engaged with the first bobbin and a second bobbin. Thus, the folding device can be used with a blind having a long shade. The smaller bevel gear presses against the larger bevel gear in a vertical direction and cooperates with the weight of the shade to provide an optional braking effect for the shade in a vertical direction.