Abstract: Transmission line condition assessment is crucial for ensuring the safe operation of power systems. Traditional single-source data source assessment methods suffer from incomplete information. Multi-source data fusion technology can effectively integrate heterogeneous data, such as electrical measurements, to provide comprehensive and accurate information support for transmission line condition assessment. This study establishes a theoretical framework for online transmission line condition assessment based on multi-source data fusion, deeply analyzing the characteristics of different data sources and their fusion mechanism. A comprehensive assessment method system, including data preprocessing, is designed, and an online real-time assessment implementation mechanism is established. Experimental verification demonstrates that this method significantly improves the accuracy and real-time performance of condition assessment, providing an effective technical means for transmission line operation and maintenance decision-making.