The crystal structure, magnetic properties and crystallization kinetics of the Fe3B/Pr2Fe14B nanocomposite prepared from lean rare-earth Pr4.5Fe77B18.5 amorphous alloy have been investigated by X-ray diffractometer (XRD), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and differential thermal analysis (DTA). The results indicate that the phases of this sample are composed of Fe3B soft-magnetic phase and Pr2Fe14B hard-magnetic phase as the alloy annealed at 650 oC for 10 min, and the grains are very small and homogeneously distributed in this nannocomposite alloys. The result of VSM analysis presents a single hard-magnetic phase behavior in this alloy. The study on crystallization kinetics for the Fe3B/Pr2Fe14B nanocomposite alloy indicates that the crystallization behavior with easy nucleation and difficult growth of Fe3B phase and with difficult nucleation and easy growth of Pr2Fe14B phase, essentially results in a stronger exchange-coupling effect in Fe3B/Pr2Fe14B nanocomposite.