Large 3D model lightweight is a practical need in CAD developments and applications. In this paper, a lightweight algorithm through intelligent geometrical information reduction for large assembly is proposed to generate simple model by using the assembly structure. This algorithm aims at reducing the size of 3D CAD model from three levels: components, feature and surfacing geometry. Key strategies such as components suppression, surface treatments based on hollow suturation and shell extraction, were used in the processing. Later experiments show that our lightweight algorithm can get satisfactory results with preferable computing performance. The lightweight technology can be widely used in many areas such as collaborative design and movement simulation, etc.