Rotor-stator axial clearance measurement requires small-size sensing probe, large measurement range, and high measurement accuracy. However, existing measurement methods cannot meet these requirements. Considering the characteristics of rotor-stator axial clearance variation, we propose a measurement method based on frequency-swept interferometry and develop a corresponding high-speed real-time online measurement prototype. The prototype combined large-bandwidth high-speed frequency sweeping and periodical Doppler error smoothing to achieve a large measurement range and a high measurement accuracy and used a single-mode optical fiber to realize the small-size probe. To test the prototype performance, we built a simulation experiment platform for the proposed rotor-stator axial clearance measurement. Dynamic clearance experimental results showed that the measurement range of the prototype was 20 mm, and the measurement accuracy was 0.08% of the full range at 15000 r/min. The proposed prototype represents a new measurement technique for rotor-stator axial clearance measurement and provides an important reference for improving measurements.