To reduce carbon emissions during heating in the manufacturing processes, microwave technology has attracted significant attention. Microwaves have considerable advantages over traditional heating methods, including more rapid heating, lower thermal damage, and eco-friendly processes. To apply microwaves to the manufacturing process, uniform and efficient heating is required. We analyzed the effect of various design parameters for uniform and efficient heating by changing the cavity heights, application of the reflector, and number and positions of waveguides. We conducted a numerical simulation and verified the findings by experiments. The results showed that a slight change in the cavity height altered the electromagnetic field distribution and heating parameters, such as the coefficient of variance and power absorption efficiency. With reflectors installed, 66% of cases exhibited better comprehensive evaluation coefficient (CEC) with consideration of uniform heating and power absorption. The spherical reflector showed 81% of cases, better than those of the ordinary model without a reflector. Furthermore, when double waveguides were installed, the average coefficient of variance (COV) was improved by 22%, and power absorption efficiency was increased by 53% compared to the single waveguide case. When the power applied to the waveguides was doubled, the average COV values improved by 18%. This large-scale analysis will be helpful in applying microwaves to actual industrial sites.