The invention relates to the technical field of medicines, discloses a compound for treating leukemia and a preparation method thereof and specifically discloses a Nelarabine crystalline compound and a preparation method thereof. The Nelarabine crystalline compound provided by the invention is different from the existing Nelarabine crystalline compound, has good solubility in water, and has stability, bioavailability, yield and purity superior to those of the existing Nelarabine crystalline compound. An X-ray powder diffraction pattern obtained by Cu-K alpha ray measurement is shown in the description. The invention also discloses a preparation method of the Nelarabine crystalline compound. The preparation method comprises the following steps: taking a Nelarabine crude product, heating and dissolving the Nelarabine crude product through a mixed solution, stirring the mixture for 1 to 3 hours, cooling the mixture to -10 DEG C, carrying out standing crystallization, carrying out suction filtration, washing the crystals through a small amount of petroleum ether twice and carrying out vacuum drying to obtain the Nelarabine crystalline compound. The preparation method is simple and is suitable for large-scale production.