The structure of rectangular microwave spouted bed was optimized based on the good electric field intensity and distribution. The evaluation index was the coefficient of variation COV which reflected the uniformity of the electric field distribution, and the average electric field intensity E_mean and the local gathering parameter E_max/E_mean. The numerical simulation of the electric field intensity in the spouted bed was carried out by using the COMSOL Multiphysics to analyze the effects of the parameters a (spouted bed width), H (waveguide position) and L (spouted bed height). The results showed that the effect of a value on COV and E_max/E_mean was the most significant, but H value on E_mean was the most significant. The interaction between a and L was significant for COV, E_mean and E_max/E_mean. The optimized structure of rectangular microwave spouted bed with single waveguide was: a=427 mm, H=200 mm, and L=1 000 mm.