Voltage source converter (VSC), owing to the advantages of their modular multilevel converters (MMC), such as simple structure, low switching losses, and the absence of harmonic filters, have found extensive applications in high-voltage direct current transmission systems (HVDC), particularly in renewable energy sources stations. At present, the operation and maintenance issues led by the online temperature monitoring of MMC sub-modules have become the focus of attention. This paper analyzes several subsystems of converter valve control and protection system, including pole control and protection system (PCP), valve control and protection system (VCP) and infrared inspection system. At the same time, based on the architecture of digital converter station, Structured and unstructured data from different subsystems is converged. The temperature related inspection data are integrated into an analysis unit to form the MMC sub-module temperature monitoring system based on multi temperature source. The analysis unit developed based on the above architecture was finally deployed in a converter station.