Atomic structures of Zr48Cu45Al7 ternary and Zr50Cu50 binary metallic glasses were investigated by performing reverse Monte Carlo (RMC) simulation upon synchrotron radiation based X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) data. Bond shortening in Zr-Al and Cu-Al atomic pairs was detected in Zr48Cu45Al7, which is attributed to strong interactions between Al atoms and its neighbors. Subsequently, their atomic structure was further analyzed by Voronoi tessellation method, it was found that all kinds of Voronoi clusters have smaller volume in Zr48Cu45Al7 than their counterparts in Zr50Cu50. Accordingly it is suggested that atoms may be more densely packed in atomic structure of Zr48Cu45Al7 ternary than in corresponding Zr50Cu50 binary MG at cluster scale, which may be the structural origin of high glass-forming ability of CuZr-based bulk metallic glass prepared by Al addition.