The invention discloses an electric performance-oriented space mesh antenna key dynamical modal selection method. The method comprises the following steps of: inputting geometric parameters, material parameters, electric parameters of modal guillotine factors; calculating an ideal antenna far electric field; establishing an antenna structure finite element model; extracting node, unit and shape function information; calculating a unit electric performance first-order coefficient matrix; grouping an overall electric performance first-order coefficient matrix; carrying out antenna structure modal analysis; outputting a modal matrix; calculating a first-order sensitivity, for the modal, of the electric performance; outputting a modal shape equivalent mass fraction; calculating significance factors, for the modal, of the electric performance; judging whether the significance factors are greater than the modal guillotine factors or not; if the judging result is positive, selecting the significance factors; and otherwise, abandoning the significance factors. According to the method disclosed by the invention, the deficiencies of the traditional dynamical modal selection method are overcome, the influences, on the electric performance of antennas, of modals in different orders are integrally considered in the modal selection process, and the problem of electric performance-oriented space mesh antenna key dynamical modal selection is solved.