With the swift development of workflow technology, workflow management system has been widely used. To date, many models and architectures are efficient in constructing workflow engine which is the core of a workflow management system. This paper presents a relational data-based lightweight workflow engine model which based on the relational database. In this model, the definition of the workflow and the data used in the workflow process are kept in the database. This model contains the necessary functions and characteristics of simple business flow rather than all the business flow. The model is composed of three main function modules. These three modules interact with database through DBMS. Under the guidance of this model, a flexible and low-cost workflow engine can be built efficiently. Now this model has been used by Shanghai Rongheng Inforamtion Technology Ltd.