Abstract: In view of the deficiencies in the existing measurement of carbonation depth of concrete， a new measuring method is presented here. The method of image acquisition (image of carbonized specimen) by using improved scanner is introduced and a new idea of using the hue gray scale image to measure the carbonation depth through comparison of every channel histogram of RGB and HSV is proposed. After that， a computer program GUI based on Matlab is developed to crop， rotate， gray and calculate the carbonation depth of concrete and exporting data conveniently. In order to verify the reliability of this new method， the carbonation depth of specimens is measured by two kinds of methods such as a conventional measuring method as well as a colour gray scale image processing method. The measured results show that the new method based on the special scanning apparatus is not only accurate for reducing human error greatly， but also convenient in operation for reducing the workload greatly.