This paper studies the key technology in the 3 - D digital city construction, and puts forward a new method to construct and realize the 3 - D digital city. The authors discuss the dates gathering and disposal from the collection of 3 - D topograph dates, the construction and ingress of 3 - D model, the gathering and disposal of texture dates. Subsequently, to solve the problem of that the 3 - D topograph dates are greater than the 2 - D topograph, the bulk-dates optimization of 3 - D digital city is realized through the choose of appropriate DEM of linenumbers and column numbers, the 3 - D model modeling specification, the using of nodes replace, the optimization of LOD modeling. The successful operation result of Chongqing Hi-tech Development Zone 3 - D digital city project illustrates that, this system can realistic mimic status in quo of the Chongqing Hi-tech Development Zone, the system operates smoothly, and its effect is also well.