The crack width on structural concrete surface is one of the most important parameters in the detection and assessment of concrete structures. At present，the contact-based method is commonly used by inspectors to detect the maximum width of crack. However, this method is a great consumption of time and energy and can not measure the maximum width of crack accurately. Based on the application of image analysis technology, a new non-contact method was proposed in this paper, which used the calibration block on the concrete surface and revised the original deformed images, thus enhancing the contrast of the revised image and smoothing them, etc. Finally, the width of crack in the image can be calculated. The method has been verified by the laboratory experiment of a cracked concrete girder using vertical and oblique photographic techniques to detect the cracks and the accuracy of 93.4% and 90.9% have been obtained, which have shown that this new method can effectively detect the width of crack.