Boiler reducing tee elbows feature complex structures with numerous interrelated dimensions. To address the inefficiency and repetitive modeling tasks in their design process, this study utilizes secondary development on the SolidWorks platform through VB.NET under the Visual Studio environment. By establishing modular assembly modeling and parametric design for critical dimensions, a parametric design system for boiler reducing tee elbows is developed. A human-machine interface enables parametric modeling of different elbow types. This modular assembly approach significantly enhances design efficiency, accuracy, and flexibility. The proposed parametric system has increased the design efficiency of elbows by more than 90% and effectively shortened the product development cycle. And optional forms such as ComboBox and CheckBox have been added to the design system, enhancing the diversity of the design.