A quick releasing device, which is adapted to be fixed in or removed from a slot of a casing, is provided. The quick releasing device includes a frame, an arm and a handle. The frame has a hole. The arm is disposed at one side of the frame and a part of the arm corresponds to the hole. The handle is rotatably disposed at another side of the frame to rotate between a working position and a retracting position. The handle includes a pressing member whose position corresponds to the hole. The pressing member of the handle passes through the hole of the frame and pushes the arm when the quick releasing device is inserted into the slot of the casing and the handle is rotated from the working position to the retracting position, deforming the arm and interfering with a wall surface of the casing beside the slot.