Abstract: At present, CATIA V6,as one of the mainstream CAD/CAE software, is widely used in the water resource and hydropower industry.However, the design model of hydropower station usually has the characteristics of large volume and inconvenient information transmission, which seriously affects the whole life cycle application of the model.Model lightweight has become an important research content of digital twin technology, so a lightweight conversion method for BIM model based on CATIA V6 is proposed in this study.Firstly, a lightweight model storage structure is defined, and the logical reference relationship between components is established by the way of "reference-instance";secondly, analytical algorithm and discrete algorithm are used to obtain model data; finally, the data will be reorganized to form a lightweight model file.Experimental results show that this method can compress the model data to the maximum extent without losing the model information.The lightweight model obtained by this method can be quickly loaded and displayed smoothly in the digital platform of engineering projects.At the same time, it can meet a variety of application scenarios, and enhance the application value of BIM models in water resource and hydropower engineering.