Relaxor ferroelectric sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) exhibits outstanding ferroelectric characteristics and is widely recognized as a highly promising lead-free ferroelectric material. In order to further promote the application of this environmentally friendly ferroelectric material, it is crucial to gain a comprehensive understanding of its structural evolution and phase transition mechanism under high pressure. This study investigates the structural evolution of NBT under hydrostatic pressure up to 6.8 GPa by integrating in situ high-pressure neutron diffraction experiments with first-principles calculations. Based on high-pressure neutron diffraction experiments conducted at the China Mianyang Research Reactor (CMRR), Rietveld refinement analysis identifies a phase transition from the ambient-pressure R3c phase to the high-pressure Pnma phase in NBT, with a coexistence pressure range of 1.1–4.6 GPa. The bulk modulus of the high-pressure phase Pnma is experimentally determined to be 108.6 GPa for the first time. First-principles calculations further support the thermodynamic tendency for the pressure-induced phase transition from R3c to Pnma and produce a bulk modulus that is in close agreement with the experimental value. By correlating with the experimentally obtained trends of the internal TiO₆ oxygen octahedral structural changes under high pressure in both phases, this study demonstrates that the difference in their macroscopic compressibility originates from the significantly higher pressure sensitivity of the oxygen octahedral distortion degree in the R3c phase than that of the Pnma phase. This relatively softer internal microstructure results in a lower bulk modulus than that of the Pnma phase. By providing a detailed analysis of the pressure-induced phase transition and microstructural evolution, this study clarifies the relationship between the microscopic structural features of the high-pressure and ambient-pressure phases of NBT and their influence on macroscopic mechanical properties, thereby establishing a fundamental connection between microscopic structural responses and bulk physical behavior under high-pressure conditions. These findings provide crucial experimental data and theoretical support for further improving the high-pressure performance and applications of lead-free ferroelectric materials.