The size-driven mechanism is only applied in automatically reconstructing parts by most of recent popular 3D CAD tools, however, most of them have not realized the automatic reconstructing assemblies at all. In this paper, we propose a novel 3D modeling method, which integrates size parameter and change rules into the model, so that an assembly becomes an object which can automatically reconstruct any parts according to the rules once the whole model size changes. Using this method, we not only keep the sizedriven mechanism in part, but also make assemblies have the ability of automatic reconstruction. The results of experiments in furniture design application show our method can greatly improve models' flexibility and reusability.