Using the series formula of superconducting critical temperature (1), we have obtained an expression of isotope effect for B-type superconductors. In both cases of μ*= 0 and μ*≠0, we have obtained a series expression for the upper limit of Tc. The agreement between the results of series formula and numerical solution is quite good. Finally, with μ*= 0, we have discussed the relation between Tc and various spectrum parameters. It is apparent from the series expression that when fixing different parameters the behaviour of Tc is completely different, hence we have made a general interpretation for the different conclusions of other authors about the effect of phonon spectrum on Tc. We emphasize that, in most cases, the series formula (1), which we took up to first two terms only, can be used as a good approximation for numerical solution,so that the series formula is simplified greatly. It may be predicted that by using this simplified formula we can investigate many other problems.