The invention relates to a method for preparing imatinib mesylate in the alpha crystal form conveniently and rapidly. The method specifically includes the steps that 4-(4-methyl piperazine methyl) benzoic acid dihydrochloride shown in the second formula and N-(5-amino-2-methyl benzene)-4-(3-pyridyl)-2-amino pyrimidine shown in the third formula serve as initial raw materials, and by using methyl sulfonyl chloride, a one-pot method is conveniently, rapidly and efficiently applied to obtain imatinib mesylate crude products; then, by re-crystallizing and adding a proper amount of seed crystals, high-purity imatinib mesylate crystals in the alpha crystal form are obtained. The preparation method is few in synthesis step, easy and convenient to operate, high in yield, high in purity of alpha crystal form of the products, low in production cost and favorable for industrialized production.