In large-scale civil engineering, the quantity of earthwork is an important parameter that affects the construction schedule and the technical and economic indexes of the project. In this paper, a precise calculation method for earthwork volume is presented based on BIM(Building Information Model) techniques to improve the calculation efficiency and accuracy. Firstly, the terrain surface is divided into several rectangular subdomains, each of which is simulated with an independent thin plate spline interpolation function. Furthermore, the terrain surface is discretized with regular right-angled triangular meshes, and the meshes are refined automatically to improve and control the simulation accuracy. In subsequence, through the topological and geometrical calculation of the terrain surface and the designed slope, the accurate excavation line is obtained and the BIM model is generated automatically. Finally, the 3D excavated body is generated by the Boolean operation between the original terrain body and the excavated slope, and the precise quantity of earthwork can be acquired. Engineering practice has proved that the quantity of earthwork in large-scale civil engineering can be obtained precisely with the present method, and the design efficiency and quality are also improved.