The present application relates to the field of pharmaceutical chemistry. The application specifically relates to a crystal form A and crystal form B of a pyrrolopyrimidine (formula I) for preparing aJAK inhibitor. The application further relates to a method for preparing the crystal form A and crystal form B, a crystalline composition comprising the crystal form A or crystal form B, a pharmaceutical composition comprising the crystal form A, crystal form B, or crystalline composition, and a pharmaceutical application of the pharmaceutical composition, the crystal form A, and the crystal formB. The crystal form A and crystal form B of the application has the advantages of high purity, high crystallinity, and good stability.