The numerical solution of the Eliashberg equation is analysed by a graphic analytic method, and the following conclusions are arrived. The superconducting Tc of a superconductor can be enhanced through the ‘hardening' of its normalized effective phonon spectrum g(ω). And the ‘softening' of g(ω) could enhance the Tc for a superconducting material with small λ, but not for the superconductor whose A is large.