Each component element of Zr57Cu20Al10Ni8Ti5 bulk metallic glass is substituted by 1 at% Ag element. Variations of glass forming ability and thermal-stability are studied using differential scanning calorimetry which gives the thermal-dynamic parameter of the bulk metallic glass, combined with X-ray diffraction of different diameter rods (Φ8, Φ10 and Φ12) which are prepared by copper mould suction casting, The results show that the glass forming ability and thermal-stability are greatly improved by substituting Ti element and the critical cooling rate is significantly reduced. While, no obvious law is found when substituting other elements. By analyzing the relevant data on glass forming ability, Inoue's atomic size rule show inconformity in this work, however, the mixing enthalpy rule dose not show conformity obviously. By calculating the packing density, we find that the packing density is obviously improved when substituting 1 at% Ti with Ag. Dynamic analysis is used and the mechanism of element substitution is also investigated on the aspects of crystallization activation energy and crystallization rate constant.