The mode of action of microwave is different from that of ordinary heat transfer. The microwave can penetrate the deep layer of the material and heat up the internal and external heat at the same time. Generally speaking, the role of the molecular polarity is stronger, more easy to absorb microwave, water is polar substance is very strong, so the water in the dry matter, because the water strongly absorbs microwave, steam can be generated in a short period of time, formed from the inside and outside temperature gradient, and the drying process with steam the migration direction and heat transfer direction are consistent, so the microwave drying characteristics of high efficiency and energy saving.