A modular heating system includes many composite heating modules, each with a microwave resonant cavity for accommodating an object. Each is separable into two parts. One part is a microwave heating semi-cavity which includes a microwave guiding device fixed on the machine frame, and the other part is a conductive heating semi-cavity which is transferrable as a carrier of heating object. The heat-receiving object is placed into a sealed container and positioned in the microwave resonant cavity. The microwaves propagated by the microwave guiding device can heat up the object. The sealed container is configured to sustain the outward expansion pressure due to the temperature rise in the object. The microwave resonant cavities act individually and do not affect each other. They are modular so as to provide the modular heating system with the flexibility to increase or decrease the module numbers for each production line.