Abstract: Compared to the success and innovation of SaaS software delivery model, SaaS software develop model need a breakthrough to meet the different needs of the changing and personalized requirement for various business domains. By applying web mashup technology, this paper proposed a software composition architecture which adds a new composition layer in web front-end. In this architecture, web pages were treated as fine grained software entities which are loosely coupled；they were designed to run in both of standalone and mashup model, and could be composited on-demand at runtime. The software entity could be provided by different providers, and could be distributed anywhere on the internet. This paper also introduced third-party contribution mechanism and application store model to enhance individual needs for SaaS application with developers’ wisdom. The prototype system could verify the feasibility and practicality of the model.