Grating-based x-ray phase contrast imaging has attracted significant attentions in the past years due to its capability in achieving x-ray phase contrast imaging with low brilliance source. The reverse projection (RP) method is a novel fast and low dose information extraction approach, which bases on the linear approximation of the shifting curve around its half-slope. However, when the refraction angle is beyond the linear range of the shifting curve, the extracted information is no longer credible. In this paper, we present an improved retrieval method by calculating an inverse function. Compared with the original retrieval method, our method does not rely on the first order approximation, and thus is suitable for large refraction angle. Theoretical derivations and numerical simulations are performed to confirm the accuracy of the method.