The expression to represent the convergence of Volterra series transfer function (VSTF) was obtained based on the series convergence theory. The relationship of the pulse′s maximum input power to guarantee the convergence of VSTF, transmission length, and the order of VSTF can be explained by the expression mentioned above. The variation of pulse′s maximum input power to guarantee the convergence of VSTF versus transmission length when using different order VSTF to approximate in the given fiber′s parameters of the circumstances was derived from the numerical simulation applied to the expression. The results shows that the pulse′s maximum input power to guarantee the convergence of VSTF is not only related to the order of VSTF applied but also the pulse′s width and fiber′s dispersion parameter.