The invention relates to a Gimeracil needle crystal (A crystal form) and a preparation method thereof, and a method for mutual transformation between the needle crystal (A crystal form) and a granular crystal (P crystal form). The preparation of the A crystal form, the transformation between the A crystal form and the P crystal form, or the purification of the A crystal form and the P crystal form adopts aqueous solution of any one of ethanol, methanol, acetone and tetrahydrofuran, or mixed solution of more than one of the ethanol, the methanol, the acetone and the tetrahydrofuran, or pure water, preferably the aqueous solution of ethanol or the pure water, and optimally preferable the pure water. By adopting the solvent in the method for crystallization, not only the transformation and simultaneously the purification of different crystal forms can be finished well, but also low-content water or even the pure water can be used as a crystallization solvent, thus the Gimeracil needle crystal and the methods have the advantages of low cost, nontoxicity, environmental protection, few organic residues and the like. Particularly the pure water is used, thus not only the purification effect is good, but also the yield is higher, and the yield is 35 percent higher than that of using absolute ethyl alcohol.