Aiming at the problem of low accuracy in calculating the width of wall cracks, a sub-pixel based crack width calculation method is proposed. On the basis of extracting the crack area, this method uses the crack width measurement method based on the vertical line of the central axis, combined with polynomial fitting the crack edge, and extracts the sub-pixel coordinate points of the left and right edges of the crack, so as to calculate the sub-pixel crack width by using the Euclidean distance method, and compares it with the pixel level calculation method. The experimental results show that the algorithm is more accurate and can be used to measure the width of various types of wall cracks. The average relative errors of longitudinal, transverse and cross cracks are 3.02%, 2.44% and 3.72% respectively. Compared with the pixel level method, the average errors are reduced by 1.87%, 1.95% and 2.07% respectively, it has strong generalization ability and stability.