The present disclosure involves a composition enriched in compound 61501b, wherein the compound 61501b has a purity of not less than 90% or more. The composition has a significant advantage in preparing a high-purity compound Sp-1. In addition, the present disclosure also provides a preparation method of the composition enriched in compound 61501b. The method adopts a crystallization technique to perform separation and purification, has a simple and convenient operation and good reproducibility, and therefore the compound 61501b in the prepared composition has high purity and quality. Further, the present disclosure also involves a novel crystal form of compound 61501b.