In order to enable the ETL system to effective achieve any data conversion between heterogeneous databases, we need a common meta-model for support the data conversion metadata customization. Through the analysis of the application of data mapping technical in the data conversion, research has been focused on the abstract study of data mapping, and defined the detailed classification of data mapping, and the form of data conversion metadata based on data mapping, build the meta-model which support to the custom data transformation metadata. The technology has been applied well in large-scale data migration of China's oil data centers.