La1−xPrxMgNi3.6Co0.4 (x=0–0.4) alloys are prepared by vacuum induction melting. The phase composition and microstructure of the as-cast alloys is characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The gaseous hydrogen absorption capacity of the alloys was measured by an automatically controlled Sieverts apparatus. The results indicate that the as-cast alloys consist of two phases of LaMgNi4 and LaNi5. The maximum gaseous hydrogen storage capacity of the La1−xPrxMgNi3.6Co0.4 (x=0–0.4) alloys is 1.768, 1.745, 1.721, 1.681 and 1.653wt%, respectively, under 3MPa H2 at 373K. But the hydrogen storage capacity after 20 cycles decays off to 0.746, 0.843, 0.947, 1.003 and 1.10wt%, respectively. In order to reveal the mechanism of rapid degradation of the capacity, the structures of the alloys before and after hydrogen absorption/desorption cycle were analyzed. SEM observation displays that the micro-cracks can be clearly seen on the surface of the alloy particles after 20 cycles. XRD detection finds that the repeated hydrogen absorption/desorption cycles give rise to a obvious broadening of the diffraction peaks of the alloys, exhibiting a typical amorphous structure, which is termed as hydrogen-induced amorphization. The La1−xPrxMgNi3.6Co0.4 (x=0–0.4) alloys after 20 cycles were annealed at 623K for 8h, finding that the hydrogen storage capacity of the alloys recovers to 1.348, 1.365, 1.50, 1.485 and 1.30wt%, respectively, under 3MPa H2 at 373K.