Abstract: Based on Petri net, a Broker-Employee architecture to deal with singularity cases in CAD systems is presented. Some potential advantages of such architecture are computability——it can be mathematically analyzed through the Petri net architecture; reusability—— it can be used repeatedly, since it is compatible with a lot of software; exchangeability——Employees conforming to standard interface specification can be mutually substituted. It can make the CAD systems more stable. Additionally, an approach to create hierarchical structure is also proposed. At last, an example is given to illustrate the mathematical analysis of our method.