The development of rare-earth permanent magnets that combine high maximum energy product with high Curie temperature has become a central challenge in the field of applied magnets. Sm-Fe-N magnets exhibit a theoretical maximum magnetic energy product comparable to Nd-Fe-B (~59 MGOe), as well as a higher Curie temperature and greater magnetocrystalline anisotropy. Furthermore, Sm-Fe-N magnets do not rely on scarce heavy rare-earth elements and are immune to price fluctuations of neodymium. These advantages position them as a highly promising rare-earth permanent magnet material, providing significant potential for achieving both high stability and coercivity. In this work, we use complementary neutron diffraction, ⁵⁷Fe Mössbauer spectroscopy, high-field magnetic measurements, and X-ray magnetic circular dichroism (XMCD) to systematically investigate nitrogen content and site occupancy, magnetic structure, and hyperfine fields, as well as the Sm/Fe spin-orbit coupling in Sm-Fe-N. The specialized sample preparation and absorption correction methods enable the acquisition of high-quality neutron diffraction patterns for Sm₂Fe₁₇ and its nitrides. The result reveals that N atoms preferentially occupy the 9e interstitial sites, forming fully nitrided Sm₂Fe₁₇N₃. By combining these measurements with the ⁵⁷Fe Mössbauer spectroscopy analysis, it is found that the nitridation reaction significantly enhances both the Curie temperature and the ground-state Fe magnetic moment, thereby improving the room-temperature magnetic properties. Furthermore, high-field magnetic measurements reveal that the anisotropy field of Sm₂Fe₁₇N₃ reaches 22.6 T at room temperature and exceeds 50 T at 2 K. This confirms the ultra-strong magnetocrystalline anisotropy of Sm₂Fe₁₇N₃, demonstrating its significant potential for achieving high coercivity. XMCD measurements demonstrate that the magnetism of Sm is dominated by its orbital magnetic moment, establishing its strong spin-orbit coupling as the physical origin of the ultra-strong magnetocrystalline anisotropy. In contrast, the orbital magnetic moment of Fe is largely quenched, resulting in the magnetic moment mainly coming from spin. This work clarifies the intrinsic relationship between the content and site occupancy of interstitial nitrogen atoms and the magnetocrystalline anisotropy, and reveals the spin-orbit coupling mechanism involving rare-earth Sm and Fe. These findings provide an important theoretical basis for designing high-performance permanent magnet materials.