1. A method for adjusting white balance, comprising steps of: displaying a selection box for a user to select an area of a preview image, so as to define a colour temperature value by the user using the area of the preview image; detecting the area defined by the selection box; acquiring a colour temperature value defined by the user using the area of the preview image; and adjusting white balance according to the colour temperature value and a preset algorithm, wherein when there are a plurality of selection boxes, colour temperature values of preview images in areas defined by the plurality of selection boxes is calculated comprehensively, and wherein an average value or a weighted average value of the colour temperature values is calculated.