Based on digital holographic tomography and transverse interference system, by means of rotating the spun polarization-maintaining optical fiber within 180°viewing angle, optimal digital holograms of a section of spun polarization-maintaining optical fiber were recorded by CCD, and then the phase information was extracted through digital image process and the angular spectrum algorithm. Using the filtered back-projection algorithm, the 2D refractive index distribution of each cross-section was retrieved in order to constitute the 3D refractive index distribution of a section of spun polarization-maintaining optical fiber, which revealed the 3D inner structure of the fiber. The measurement results show that the stress legs rotate synchronously with the spin of polarization-maintaining optical fiber, and the refractive indices of the fiber core, stress legs and cladding are unchanged. Finally, the position of the stress legs was achieved by edge detection algorithm and then the spun angle was computed.