In this paper, the problems of calculating and solution about electromagnetic fields were studied. An accurate theory model was established which can apply in high-performance numerical calculation. The new calculating method was proposed using Maxwell's equations in the circumstances of that electromagnetic field is static, contains axial symmetric charge distribution, finite, differentiable and integrabel. This calculating method was called alternating iterative method. The expression form was given on the basis of the calculating method. The electromagnetic field outside of axis can be expressed as each order derivative and one-dimensional integral of electromagnetic field which on the symmetric axis. By means of this method, the expression form is progression, which is the best form use for computer to conduct approximate calculation. It is very easy, quickly and accurately to carry out numerical calculation by using this method. The calculation method has important theoretical significance and broad prospect of application.