The invention discloses an antitumor medicine, namely, a nelarabine compound, belonging to the technical field of medicines. The nelarabine compound is a crystal, and an X-ray powder diffraction pattern tested by using Cu-Kalpha rays is as shown in the figure 1 in the specification. The novel crystal of nelarabine, which is provided by the invention, is of a crystal form structure different from that of the prior art, tests show that the compound of the novel crystal form is high in purity, good in mobility and water solubility, high in stability, not liable to absorb moisture, and safe and reliable in clinical application.