Fast auto-focusing technology is a crucial technology in computer vision and digital video system. A novel rapid auto-focusing method is proposed according to the model of optical defocusing system by using autocorrelation of derivative image. Firstly, a Laplacian operator is used to search an isotropic second-order differential to the defocused image and get the autocorrelation of the derivative image. Then, the radius of point spread function, that equals the blur′s radius of the defocused image, can be obtained from the annular groove of the autocorrelation. Finally, the right focus position can be deduced from the geometric relation between the radius of blurred spot and the defocusing value. The experimental results show that this proposed method can accurately estimate the radius of blurred spot and the right focus position for any target. Compared with the traditional methods based on focus criteria functions, this method only needs 2 defocused images that reduce the computation quantity and simplify the system to meet the fast auto focusing requirement.