By using numerical method, the superconducting critical temperature Tc is cal-culated from the Eliashberg equation and the dependence of Tc on λ and the shape of effective phonon spectrum is investigated. In this study, α2F(ω) is taken as the double delta-function spectrum and the spectrum parameters are permited to change over a wide range. It is found with surprising that in the regime of λc series solution, Tc depends, not only on λ and ratios of moments but also on Λ, the reciprocal of the convergence radius of the Tc series solution, and the relation between them shows a certain regularity. Under the light of these results, an approximate Tc formula for the case of μ*= 0 is constructed in the regime of λ 0, or Λ is greater than λ0 but λ is smaller than λ0. Here, λ0 is a parameter depending on the shape of spectrum and its definition is given in this paper. For other type A.superconductors (λ0c is determined is not λ, but δ=1/∧0.5(ω1/2/ωlog)5.5λ1.55.