Based on the interference fringe visibility of the ultra-small GRIN fiber probe, we study the influence of the GRIN fiber probe on the F-P interference signal. Furthermore, we analyze a theoretical formula of the fringe visibility of the ultra-small GRIN fiber probe based on the Gaussian beam and establish a corresponding test experimental system for verification. The experimental results show that for the ultra-small GRIN fiber probe with a focal spot diameter of 15 µm, the fringe visibility remains above 0.8 within a cavity length of 600-1350 µm. However, for single-mode fiber probes, the fringe visibility remains at 0.8 within a cavity length range of 130-530 µm. Therefore, the proposed ultra-small GRIN fiber probe improves the fringe visibility of the F-P interferometer for a long cavity length. This paper provides the theoretical foundation for further research into the application of the ultra-small GRIN fiber probe, which requires a long initial cavity length or a large dynamic range.