Epalrestat C crystal form and preparation method thereof

The invention provides an epalrestat C crystal form. Cu-K alpha radiation is employed, and X-ray powder diffraction expressed by 2theta angle has characteristic peaks at 7.930+/-0.1, 12.663+/-0.1, 16.086+/-0.1, 16.539+/-0.1, 17.341+/-0.1, 19.645+/-0.1, 21.433+/-0.1, 22.100+/-0.1, 22.894+/-0.1, 25.458+/-0.1, 27.087+/-0.1, 30.083+/-0.1, and 33.369+/-0.1. The epalrestat new crystal form provided by the invention has the advantages of simple preparation, high crystal form purity, and low content of impurities. The finished product epalrestat produced by the prior art has a Z-isomer content of 0.24-0.26%, and the finished product epalrestat prepared by the refining method provided by the invention has a Z-isomer content of 0.04-0.07%. The finished product epalrestat produced by the prior art has a total impurity content of about 0.3%, and the finished product epalrestat prepared by the refining method provided by the invention has a total impurity content of less than 0.1%. The method provided by the invention has the advantages of simple operation and good reproducibility, and is suitable for industrial production.