With the increase of the customer demands for the products, multidisciplinary products are gradually becoming more competitive than the traditional products. Considering conceptual design synthesis is the core phase of the product design and its result is the starting point of the next detail design works, if the rich resources in multiple disciplinary domains can be fully used during this phase, the efficiency of multidisciplinary product design will be largely promoted. Therefore, we proposed a novel conceptual design synthesis system for multidisciplinary products, which consists of three parts, i.e. basic framework, complex-number-domain-based mathematical model, and achieving approach. In the framework, three groups of concepts were defined and the conceptual design synthesis was concluded into three steps. The model can completely describe and modularize the function units which are the basic components of a design scheme. Based on the model, the achieving approach can automatically generate stable objective function unit chains which can directly construct the design scheme. Finally, the design of a multifunctional agricultural hydroelectric power system was taken as an illustration to prove the practicability of this proposed system.