Large-scale component assembly, such as the installation of aircraft wings and ship section etc., is usually high cost, long cycle. Virtual assembly simulation gives the possibility of direct and visual perceive of assembly before the actual assembly using digital prototype. A new virtual assembly model, called VAFM (Virtual Assembly Fat Model), is presented. It integrates not only geometry data, but non-geometry information is embedded. All information of model will evolve, mature and derived gradually, from thin to fat, during the whole assembly process. The paper gives the definition of a VAFM, its framework and the evolution rules of the fat model. Meanwhile the paper design the modeling language based XML. Finally, the visualization of VAFM is also presented..