FIG. 1. Allowable range of ( M $^{2}$,s$_{0}$ ) for sum rules I, where m$_{b}$ = 4 . 68 GeV and the non-perturbative condensates are set to be their center values. The left diagram is for criteria (A) and (B), whose third axis is for the ratio of the continuum contribution and the dimension-six contribution over the total contributions, respectively. The right diagram is for criterion (C). FIG. 2. Allowable range of ( M $^{2}$,s$_{0}$ ) for sum rules II, where m$_{b}$ = 4 . 68 GeV and the non- perturbative condensates are set to be their center values. The left diagram is for criteria (A) and (B), whose third axis is for the ratio of the continuum contribution and the dimension-six contribution over the total contributions, respectively. The right diagram is for criterion (C). µ$_{IR}$ = √ m 2 B − m 2 b [28]. The b -quark pole mass is taken as: m$_{b}$ ≃ 4 . 68 +0 . 17 − 0 07 GeV, which is the recent world average suggested by Particle Data Group [31]. As for the running coupling constant, we approximate it by a one-loop form, α$_{s}$ ( µ$_{IR}$ ) = 4 π/ (9 ln( µ 2 $_{IR}$/ Λ 2 $_{QCD}$)), where Λ$_{QCD}$ = 0 . 241 GeV that is determined by reproducing α$_{s}$ ( M$_{z}$ ) = 0 . 1176 [31]. Criteria (A, B, C) determine a set of parameters for each value of m$_{b}$ . In the following, we shall firstly take m$_{b}$ = 4 . 68 GeV as an explicit example to show how to determine the allowable ranges for M 2 and s$_{0}$ and to derive f$_{B}$ within these ranges. And then we shall present the results for several typical m$_{b}$ . Firstly, the three-dimensional figures (1,2) present the allowable range of M 2 and s$_{0}$ for sum rules I and II. The left diagram of Figs.(1,2) is for criteria (A) and (B), whose 2 4 6 8 M 2 32 33 34 35 36 s$_{0}$ 0 0.1 0.2 4 6 8 M 2 32 33 34 35 36 s$_{0}$ 5.27 5.275 5.28 5.285 m$_{B}$ 2 3 4 5 6 M 2 32 33 34 35 36 s$_{0}$ 0 0.1 0.2 2.5 3 3.5 4 M 2 32 33 34 35 36 s$_{0}$ 5.27 5.275 5.28 5.285 m$_{B}$ 6