Topological pipe-net models of engineering CAD were established using templates data library. The topological templates data library was constituted of many topological models each of which was abstracted from engineering object and had its unique characters. The loosely coupled three-tier architecture system that was applied to model pip-net models of engineering CAD was designed based on topological templates data library. The system included three layers: user layer, templates data library management and data layer. The templates data library management provided convenient and instant management of topological templates, and consequently the establishment and maintenance of pipe-net templates became more efficient. The data layer provided common data interfaces and thereby the topological templates data from different fields could be shared. The system hierarchy is clear, and the dependent relationships between system models are fewer, and especially it is easy to be realized. The efficiency and compatibility of pip-net modeling in engineering CAD field can be greatly improved using this system.