A test system including light path system and software system is built for measuring the refractive index of fiber by using the digital holographic tomography technology. Based on the phase distribution curve of object wave the optimal hologram is recorded. The spectral range corresponding to the object wave is selected accurately based on the optimization of the spectrum. The phase distribution information of object wave is extracted with the angular spectrum reconstraction. The accuracy of extracted phase information has been verified through the simulation holograms of optical fiber. Combined with the multilayers model of fiber, the refractive index distribution along the diameter direction of fiber tomogram is recovered from a single hologram. Single-mode and multi-mode optical fiber are tested as samples. The experiment results are consistent with the results of S14 refractive index profiler, the precision is 10-4. The compare result shows that our method is simple, fast and accurate for the measurement of the refractive index of fibers. We have also studied the measurement of the refractive index distribution of the special optical fiber.