A kind of optical fiber F-P sensor was studied for the control of the cavity length consistency. The cavity length was controlled by three-dimension frames. On the basis of the F-P cavity principle, the F-P cavity was under real-time detection by spectrum analyzer and then was adjusted. The F-P cavity was composed of hollow glass tube and optical fiber with the method of thermal fusion. The key parts of the processed F-P cavity were examined using optical microscope, and the fusion points were examined on their robustness. The results show that the F-P cavity length is better controlled and the consistency is good. The fusion areas are little out of shape and it has few influences on the F-P cavity′s performance.