The digital image/speckle correlation method (DSCM) is a useful nondestructive testing method in the whole displacements and strains. Designing accurate sub-pixel searching algorithms is an important problem in order to enhance the accuracy of measurement. The principle of the digital image/speckle correlation technique is studied, and a kind of gradient method with changeable step is proposed and applied to the correlation searching. The reliability and stability of this algorithm is proved by the displacement analysis of the computer-simulated speckle images, and the translation of the rigid is measured, the results indicate that this algorithm is a stable and reliable, effective searching algorithm, and it can be used in the measurement of displacement fields.