Abstract: For the problem of uneven electric field distribution in the microwave reaction chamber with high power closed-field characteristics, this paper designs a microwave reaction chamber with controllable metamaterial, uses the electric field distribution diagram method and the standard deviation analysis method of electric field distribution sampling to research the load effects of size, heating frequency, and controllable metamaterial parameters on heating efficiency and heating uniformity. The results show that by way of properly adjusting the parameters of the controllable metamaterial, the heating uniformity can be improved up to 48% on the premise of ensuring at least 97% heating efficiency.