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Principle of atomic force microscope (AFM) is on the basis of cantilever's deflection. However, up to now, there are still no effective methods to model the cantilever's deflection with high precision, which will usually result in a poor measurement accuracy of AFM and has greatly limited further applications of AFM in more different fields. Thus, a global shape from defocus method, which is only based on a single vision sensor, is introduced in this paper to reconstruct the bended shape of AFM cantilever. First, the model of the defocus imaging is given using the concepts of relative blurring and diffusion equation. Second, the relationship between the relative blurring and the interested depth information is built with basic imaging formulas. Subsequently, the depth measurement problem is transformed into an optimization issue and an algorithm is designed to compute the deflection of cantilever. Finally, extensive experiments are conducted and results are analyzed to show the feasibility and the effectiveness of the proposed method.