A cavity forming method includes the steps of: providing the material modification processing device; according to the cavity topography of the workpiece, utilizing the material modification processing device to perform local modification including: calculating the laser-light shaping and scanning information, and based on the laser-light shaping and scanning information to have the optical axis adjustment unit to adjust positions of the laser-light shaping and scanning processing module and the processing stage, such that the area of the workpiece to be projected by the Bessel beam can be formed as the modified area; and, etching the modified area to form a cavity of the cavity topography. In additional, a material modification processing device is also provided.