The method for interpolating the dependence of the electron beam radius on the cutting plane position by the longitudinal coordinate, based on using the special class of interpolation functions, was investigated in this article. The roots of n-th order from the polynomials of the same order are used as the class of interpolation functions, which correspond to the ravine dependence of the radius of a short-focus electron beam on the distance of propagation. The results of testing have shown that, depending on choosing the set of basic points, including its position relatively to the minimum and their quantity, the relative interpolation error is between 0,7% and 8%. Obtained results of mathematical simulation of focal parameters of electron beams have the significant practical importance for the specialists, involved in the development and introduction into industrial production of modern highly effective electron-beam technological equipment.