A method for fabricating MEMS device includes providing a silicon substrate. A structural dielectric layer is formed over a first side of the silicon substrate. Structure elements are embedded in the structural dielectric layer. The structure elements include a conductive backplate disposed over the silicon substrate, having venting holes and protrusion structures on top of the conductive backplate; and diaphragm located above the conductive backplate by a distance. A chamber is formed between the diaphragm and the conductive backplate. A cavity is formed in the silicon substrate at a second side. The cavity corresponds to the structure elements. An isotropic etching is performed on a dielectric material of the structural dielectric layer to release the structure elements. A first side of the diaphragm is exposed by the chamber and faces to the protrusion structures of the conductive backplate. A second side of the diaphragm is exposed to an environment space.