The relationship between defocus and the lateral shifts of cubic phase mask has been built based on the Fresnel integral. Based on the fact that the artifacts appeared in the decoded images are resulted from the differences between the coding and decoding point spread function, the artifacts can be treated as indicators of defocus. Finally, we combined the built relationship previously and the reason producing artifacts in the decoded images to propose an imaging way to obtain defocus map and artifacts-free images with laterally shifted cubic phase mask, which subsequently verified by experiments with our designed imaging system. The proposed method avoids errors introduced by shifting the ideal imaging plane deliberately. Moreover, although the idea we developed only was analyzed and illustrated with cubic phase mask, it also can be generalized to other types of odd-symmetric phase masks, which further widen the application range of odd-symmetric phase mask imaging system.