Pyrrhotite formed in pseudotachylyte is an important magnetic mineral for understanding earthquake mechanisms and seismogenic environments. To reveal the formation mechanism of pyrrhotite within pseudotachylyte, we conducted microstructural, geochemical, and rock magnetic (after heating) experiments on samples from the Wenchuan Earthquake Fault Scientific Drilling project Hole 2 (WFSD-2), in the Longmen Shan thrust belt on the eastern margin of the Tibetan Plateau. Metallic iron was formed by the thermal decomposition of iron oxide and sulfide at high temperatures (≥1300 °C) within a reducing environment. Then, monoclinic pyrrhotite was formed at a very high melting temperature (≥1500 °C) from the reaction between a limited amount of sulfur and metallic iron. Metallic phosphorus was formed at temperatures ≥1300 °C and was preserved as Fe-P alloy. We observed that the quantity of Fe-P alloy spherulites increased with increasing temperature, as did the complexity of their structure and composition. Isometric polyhedral Fe-P alloy spherulites began to form at 1500 °C. The spherulites that formed at 1750 °C had a more complex structure and a perfectly regular shape compared to those formed at 1500 °C. We suggest that metallic phosphorus from fault zones, along with carbon and metallic iron, could be a new indicator of large earthquakes. The monoclinic pyrrhotite and excess iron in the pseudotachylyte from WFSD-2 indicate a frictional temperature of ≥1500 °C and a strongly reducing seismogenic environment.