Based on classical electrodynamics theory, the theoretical prediction equations for calculating the thrust of propellanless microwave thruster is deduced according to Maxwell equation and tress tensor. With the calculated electromagnetic fields by using finite element analysis software, the electromagnetic field distribution of a defined thruster cavity operating in a specific resonant mode under different electromagnetic powers is calculated to predict its thrust according to the deduced method. It is found that the thrust increases with the increase of the electromagnetic power and the total thrust is determined by the magnetic thrust. The net thrust will rang from 20 to 250 mN in a power range from 20 to 200 W.