A method for manufacturing a MEMS sensor and its thin film and cantilever beam includes steps of etching a top surface of a single-crystal silicon wafer in combination of a deposition process, an outer epitaxial growth process, a wet etching process and a back etching process in order to form a pressure-sensitive single-crystal silicon film, a cantilever beam, a mass block, a front chamber, a back chamber and trenches connecting the front and the back chambers. The single-crystal silicon film is prevented from etching so that the thickness thereof can be well controlled. The method of the present invention can be used to replace the traditional method which forms the back chamber and the pressure-sensitive single-crystal silicon film from the bottom surface of the silicon wafer.