The invention relates to nelarabine crystalline compound and a preparation method thereof. The crystalline compound is measured by a powder X-ray diffraction method; and the obtained X-ray powder diffraction pattern is shown in a figure 2; the nelarabine crystalline compound is a novel crystal form which is different from that in the prior art; and the crystal form has obviously improved dissolubility in water. The invention also relates to a preparation method of the crystalline compound. The preparation method comprises the following steps of: taking and putting a nelarabine crude product into absolute methanol, heating and refluxing; adding active carbon for refluxing after dissolving the nelarabine crude product; filtering at high temperature, cooling to room temperature, cooling by virtue of ice salt bath, devitrifying and performing suction filtration, and washing a filter cake by ice methanol to obtain the nelarabine crystalline compound. The method disclosed by the invention is simple and convenient to operate, and is a method which is economical, and feasible, and suitable for industrial purification.