Abstract: Based on the Finite Integration Theory（FIT）, a simple model has been developed to simulate the electromagnetic field and plasma densities in a rotational symmetric microwave-reactor.The computer programs calculate alternately the electric field for a given spatial distribution of the plasma and the plasma for a given distribution of the electric field.A self consistent solution of electromagnetic field in the microwave field plasma system and the location of discharge of plasma have been obtained.Considering some adjusted parameters in the experiments,the parameters which effect the location of discharge of plasma have been discussed. So the design of the microwave plasma reactor for large area diamond CVD can be predicated.