The present invention relates to novel mesylate salt of N-(5-(4-(4-((dimethylamino)methyl)-3-phenyl-1H-pyrazol-1-yppyrimidine-2-ylamino)-4-methoxy-2-morpholinophenyl)acrylamide, a novel crystalline form thereof, and a process for preparing the same. More specifically, the present invention relates to mesylate salt of N-(5-(4-(4-((dimethylamino)methyl)-3-phenyl-1H-pyrazol-1-yl)pyrimidine-2-ylamino)-4-methoxy-2-morpholinophenyl)acrylamide, which is excellent in stability, solubility, and bioavailability when it is administered not only alone but also in combination with other drugs and which has a high purity, a crystalline form thereof, and a process for preparing the same.