Urban underground space resource (UUSR) assessment can facilitate the planning of underground space development and utilization in highly populated urban areas. Typically, UUSR assessment involves dividing the assessment area, determining the assessment index, establishing the assessment model, integrating the assessment data, and calculating the results. A method for streamlining the three-dimensional (3D) high-precision UUSR assessment process was proposed, aiming at the mountainous cities. Assessment indexes suitable for the mountainous UUSR were determined, and a variable weighting model was constructed to enhance the credibility of the assessment results. Multi-source assessment data were compressed and stored using a chain run-length encoding data structure, which can be queried to obtain the attributes of the assessment cells. In addition, a 3D assessment system was developed to automate the assessment process using distributed computing. Finally, taking Chongqing City as a case study, the assessment was conducted over a surface area of approximately 19.4 km² and to a depth of 50 m. The study area was divided into 9.72 × 10⁸ assessment cells, with the dimension of 1 m × 1 m × 1 m. The results show that the 3D high-precision UUSR assessment successfully reported the construction risks at the urban scale. The construction risks of a 15-km underground rail transit line can be analyzed, thereby providing decision-making support for underground space development and transit line layout.