According to the E-commerce demands of small and medium sized companies, this paper puts forward a scheme for the border construction of third party E-commerce platform. It introduces the tag warehouse based on the ASP.NET(C#) and the design and realization of the tag analyzing engine, demonstrating the feasibility of the scheme through the development of a website building system. Besides the function of website building, we can extend the platform of third party E-commerce by adding new functions using tags.