With increase of machine speed, the influence of elastic components on dynamic characterises has become a very challenging problem. In our paper, we use self-made fiber-optic sensor on-line to detect the elastic locus. The sensor has highly transmitting and collecting light properties so that steady elastic locus has high precision and distinguishabilty. A theoretical analysis has been conducted using our program KEDL. The conclusions reveal more information about the dynamic characteristics of high-speed mechanisms.