An assembling device for assembling an electronic device which includes a front shell, a rear shell, a lens cover mounted on one of the front and rear shells includes a supporting member and a pressing member. The supporting member includes a base and a positioning block according to the lens cover to be assembled. The base defines a well for receiving the electronic device to be assembled. The positioning block is positioned in the well and has an adjustable height relative to the bottom surface of the well. The pressing member includes a pressing portion which is configured for pressing one of the front and rear shells of the electronic device. Thereby when assembling the electronic device, the height of the positioning block can be adjusted to press or not press the lens cover depending on which of the front and rear shells is assembled.