To objectively evaluate the geological suitability of underground space development and provide a new approach and reference for evaluation work, an evaluation method combining FAHP (fuzzy analytic hierarchy process) and TOPSIS (technique for order preference by similarity to ideal solution) was proposed. Based on geological survey research results, an evaluation factor set was established, primarily encompassing rock and soil engineering geological conditions, hydrogeological conditions, adverse geological processes, and terrain and landforms. FAHP was employed to determine factor weights, and TOPSIS was innovatively applied to determine non-uniform suitability classification. Given that interval numbers were primarily used to express evaluation factors, a new TOPSIS method based on an improved interval number approach was developed, and an evaluation model was constructed using this new method. Then, the evaluation area was divided into multiple evaluation unit grids using ArcGIS, the suitability rating of each unit grid was calculated through evaluation model at the same time. Compared with traditional methods, this method reduces the subjective influence of expert to some extent. The evaluation process are more quantitative, and the evaluation results are more objective. The geological suitability of shallow underground space development in Wuxi City was evaluated using this method, and the evaluation results were consistent with actual engineering experience, proving the effectiveness of this method. Therefore, this method has certain reference significance for the suitability evaluation of underground space development.