<p>To solve the problem of a lack of crustal stress field data in some underground projects, an estimation method for the crustal stress field is proposed. After reaching the yield point under a low confining pressure on a hard brittle rock mass, its strength rapidly decreases to a lower value after minimal strain. Under the unloading effect, the surrounding rock of an underground engineering tunnel is subjected to low confining pressure within a certain depth range. After reaching the plastic state, it is likely to lose stability under an excavation disturbance. Therefore, the surrounding rock that loses stability around a disturbed tunnel near the slope is regarded as the plastic zone, and numerical simulation is employed to estimate the crustal stress field by fitting under various crustal stress field environments. Taking the Wawu IV ore section of the Xingshen Phosphate Mine as the study area, the instability and failure data of the tunnel’s surrounding rock near the stope were collected and the macro crustal stress field was analyzed. It was concluded that the tectonic stress direction in the project area was approximately N45°W. The results of this study can serve as a reference for mine production and construction.</p>