In this paper, the basic ideal of the Event Space Theory and Analyzing Events are expatiated on. Then it is suggested that how to set up event base library in developing application software. Based above the designing principle of facing methodology. Finally, in order to explain how to apply the Event Space Theory in developing economic evaluation software, the software of "sewage treatment CAD" in a national "8th-Five Year Plan Research Project" of PRC is used as an example. This software concerns economic effectiveness evaluation for construction projects.