The effects of composition changes of major elements on glass forming ability and microstructure in Cu50-xZr40+xAl5Nb5 (x=0, 2, 4, 6) alloy system were investigated. And the relationship between microstructure and mechanical properties were studied. The results show that Cu46Zr44Al5Nb5 with good thermal stability and best glass forming ability in the alloy system, whose ΔTx, Trg and γ are up to 51.9 K, 0.60 and 0.401, respectively. The fracture strength of Cu46Zr44Al5Nb5 amorphous alloy is up to 1811 MPa with no obvious macro-plastic deformation. And the fracture surface is comparatively smooth, with the vein-like patterns widely distributed in the same extend direction. The Cu44Zr46Al5Nb5 alloy with an amorphous and crystalline composite structure exhibits good comprehensive mechanical properties including the fracture strength of 1747 MPa and plastic strain of 3.94%. And its vein-like patterns on fracture surface are closer and deeper. The shear bands width is about 40 μm with the axial compressive direction about 45°. The crystalline phase of Cu44Zr46Al5Nb5 alloy is well-distributed, the size of which is smaller than the shear band width, toughening the glass matrix as loading. The microstructure, size and distribution of the crystalline phase determine the mechanical behavior of the composites