The invention relates to a method for monitoring the width of the crack of a concrete structure surface by using an unmanned aerial vehicle. The method comprises the following steps that: a, a scale mark generating device is installed on the unmanned aerial vehicle; b, the unmanned aerial vehicle is driven to fly on a target concrete structure surface (2), when the normal camera of the unmanned aerial vehicle finds a crack (3), the unmanned aerial vehicle hovers before the crack (3); c, the scale mark generating device of the unmanned aerial vehicle acts so as to make a scale mark appear near the crack (3) of the target concrete structure surface (2); d, the high-definition camera of the unmanned aerial vehicle is adopted to directly face the crack (3) to take the photo of the crack (3) and make the crack (3) and the scale mark appear in the same photo; e, the photo is processed on a computer by using Photoshop software, and the scale mark is moved onto the crack (3); and f, and the width of the crack (3) is directly read according to the scale mark. With the method adopted, the actual width of the crack (3) can be accurately measured.