Abstract: The modeling of multi-domain systems based on Modelica mainly regards such aspects as physical modeling and parameter optimization,whereas it does not support the product variant design.By extending the model expression,a variant product model with undefined components or parameters is presented to support dynamic variant design of multi-domain systems.Knowledge and experience of product design are fused in the model in the manner of constraint rules with regard to the parameters,components and models.The components and parameters of the products are determined through reasoning so that dynamic variant design is achieved.At last,an application example is given to assess the effectiveness of the method.