The complexity evaluation for geological conditions of foundation pit is characterized by fuzzy and random. Hence, a method for determining the cloud model of complexity evaluation of geological conditions of foundation pit with quantitative index as interval number is established based on fuzziness and randomness of cloud model. The geological characteristics of Fujian area and possible risks during foundation pit construction are analyzed and summarized. The complexity evaluation system for geological conditions of foundation pit and quantitative standard of each index are established. The evaluation grades are divided into 4 kinds, i.e. simple, general, relatively complicated and complicated, which respectively correspond to intervals of ［0,2.5), ［2.5,5.0), ［5.0,7.5) and ［7.5,10.0］, and the numerical characteristics of corresponding cloud model is determined. The abovementioned theory and method are applied to a foundation pit of Fuzhou Metro Line No. 2, the treatment measures for groundwater and soft soil are put forward based on evaluation results, and the feasibility and effectiveness of abovementioned method are verified by engineering practice.