In the World Wide Web, a very large number of online cultural heritage (CH) resources is made available through digital museums websites. They display collections images and share collection metadata, which makes opportunities for aggregating. 'China Art in the Museums Overseas' platform aggregated China digital collections from overseas institutions. This paper introduces the metadata aggregation workflow of this project, especially a unified data model used to solve the metadata standards heterogeneity. Besides, this platform is not simply a database, but also provides search, images semantic annotation, knowledge graph serves, linked open data interface and so on. Our work is an effort to resolve multi-source data aggregation and it is valuable. It improves the availability and discoverability of digital collections, and spread Chinese culture to the public more widely.