The present invention relates to a crystalline form of a BTK kinase inhibitor and the preparation method thereof. In particular, the present invention relates to a crystal form I of (R)-4-amino-1-(1-(but-2-ynoyl)pyrrolidin-3-yl)-3-(4-(2, 6-difluorophenoxy)phenyl)-1H-pyrrolo[2, 3-d]pyridazin-7(6H)-one (the compound of formula (I)) and the preparation method thereof. The crystal form I of the compound of formula (I) obtained by the present invention has a good crystalline stability and chemical stability, and the crystallization solvent used has a low toxicity and low residue, thus making it more suitable for use in clinical treatment.