A method of coupling waveguide slot's active impedance calculation is presented due to the complicated electromagnetic field in waveguide and disadvantage in engineering application.The equation contained slot's active impedance is created according to the relation between the S matrix and the input impedance at the port of coupling waveguide.Newton method is applied to solve the higher order fractional equation to obtain the coupling slot's active impedance which takes mutual coupling between slots into account.Computer simulation results obtained by applying S matrix extracted from HFSS software in coupling slot's active impedance formula indicates that the method proposed in this paper is effective and suitable for coupling slot of large waveguide slots array antenna design.