The development of low-code applets is an important means for traditional enterprises to achieve their digital transformation. The low-code applets reduce the demand of clients for code development technology by providing them with a visual application development environment, and construct a digital operation mode to further accelerate the pace of enterprise refinement construction. This paper analyzed the development trend of seamless replacement of fusion templates for low-code applets, and used development technologies such as cascading style sheets (CSS) and JavaScript to implement the design of seamless replacement of templates. Based on the three-layer architecture of presentation layer, business logic layer and data access layer, as well as the "Model-View-Controller (MVC)" model, the author studied the seamless replacement of fusion templates for low-code applet site builders, and verified the builder functions through black-box testing, realizing the seamless replacement of templates.