The key step of exploiting new type BMG is to judge quickly the glass forming ability (GFA) of bulk metallic glasses (BMG). The reliability and limitation that T_rg was suited for judging of the GFA of BMG was made clear by using dynamics based on the addition principle in this paper, and the theory basis was provided for making BMG. The minimum of T_rg for forming the BMG was 0.4066, at the same time, two theory optimum T_rg, i.e. T_rg = 1, T_rg = 0, were obtained. A new parameter (CPS) for judging the magnitude or the smooth and stability of GFA of BMG was put forward, the magnitude of GFA of BMG was calculated by using it. The result of sequence of GFA of BMG was unanimous to Inoue's. At one time, the results were examined according to the average variance of Z_max and R_c of BMG, and all results were measured with one another. The magnitude of AH_mg was a feasible method for quickly appraising the capacity and stability of GFA of BMG.