An electronic device used in a method for expanding image depth obtains first images by a first sensor, the first images comprising depth information. The electronic device obtains second images by a second sensor, the second images comprising gradient information, and the first images correspond to the second images. The electronic device determines the pixels in the first images which contain expandable content according to the gradient information of the second images, applies expansion accordingly to the pixels in the first images to generate third images, and generate target depth maps according to the gradient information of the second images and the depth information of the third images.