This paper proposes a dynamic model for measuring the 3D tip clearance of rotating blades using an isosceles-right-triangle sensing structure consisting of three identical two-circle coaxial optical fiber bundles and a demodulation approach using the ratios of difference signals between any two bundles based on the Taylor expansion principle. The dynamic system comprising the optical fiber probe, hardware circuits, and measurement software is designed and established according to the characteristics of dynamic sensing signals. Finally, the feasibility of the system for the dynamic measurement of 3D tip clearance is experimentally verified using the simulated blades of a rotor test bench.