Abstract: As a micro-nano machining method, the focused ion beam (FIB) processing can be used to manufacture the nanometer elements and microstructure elements. The whole process of designing and manufacturing the 45° mirror surface at the end of multi-core fiber using focused ion beam processing technology was studied. This mirror surface was manufactured in two steps: the first was the scanning process, which was used to manufacture the rough cut surface; the second was the polishing process, which was used for the surface finish of optical surface. The machined 45˚ mirror surface could be accurately aligned with the fibre core of optical fiber, which avoided related issues of external turning mirror components docking. The experimental test show that the displacement value in two vertical axial directions can be measured by the machined structure with interferometry. The measuring range of the detection displacement is approximately 60 µm, and the absolute measurement error of root-mean-square in X and Y axial directions is 1.75‰ and 1.97‰, respectively. This technology is expected to be used in the detection fields of inner surface and vessels inter wall for precision components.