A micro-electro-mechanical system (MEMS) microphone module and a manufacturing process thereof are described. A thickness of a transparent temporary cover plate temporarily disposed in a conventional plastic package structure is adjusted. After a mold for a plastic protector is formed, an UV ray is utilized to irradiate the mold to reduce adherence on the temporary cover plate and a back surface of the MEMS acoustic wave sensing chip. Then, the temporary cover plate is removed, and the left space left is the main source for the back-volume of the MEMS microphone. Finally, a tag is covered on the plastic protector, so as to define the whole back-volume and form a closed back-volume. In the above-mentioned process, the size of the back-volume is the same as an area of the whole MEMS microphone chip. In addition, the back-volume can be defined.