To make the online transient early warning and security protecting system easy to reuse and maintain， a software development pattern is proposed. Several advanced methods， such as architecture analysis， design pattern， framework and unit testing， are used in this pattern to enhance the development efficiency and software quality. A layered architecture is introduced during the design process and a“strategy pattern” is used to adapt to the changing requirement for the dynamic model of the component concerned. An open framework for distributed task dispatching is also presented. A“subscriber/publisher” pattern is used to implement message exchange between the server and clients， which is adaptive to the hardware configuration changes. An online transient early warning and security protecting system based on the methods proposed has been developed and put into actual operation in a provincial power system.