In order to design an amorphous hydrogen storage alloy efficiently, the maximum hydrogen capacity of the amorphous alloy Zr-Ni has been calculated. Based on the Rhombic Unit Structure Model (RUSM) of the amorphous alloy, and the experimental result that hydrogen atoms exist in 3Zr1Ni and 4Zr tetrahedron interstices in the amorphous alloy Zr-Ni, the numbers of 3ZrlNi and 4Zr tetrahedron interstices in a RUSM, which correspond to the hydrogen capacity, have been calculated. Two extreme Zr distribution states (highly heterogeneous Zr distribution and homogeneous Zr distribution) have been calculated. The calculated curves of the hydrogen capacity with different Zr contents at two states indicate that the hydrogen capacity increases with the increase of Zr content and reaches the maximum at a Zr content of 75 at%, The theoretical maximum hydrogen capacity of the amorphous alloy Zr-Ni is 2.0 (H/M). The hydrogen capacity of heterogeneous Zr distribution alloy is higher than that of homogenous one for the same Zr content. The experimentally determined hydrogen capacity of the amorphous alloy Zr -- Ni lies between the two calculated curves, which proves that the calculated results reasonable. In this way the heterogeneous distribution of Zr atoms in amorphous alloy after a few hydrogen absorption-desorption cycles can be explained.Correspondent: Zhang Dongming, Associate Professor, State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China, Tel: 086-27-87884448