As a key component of the flexible DC converter valve, DC support capacitor is very important for the stable operation of the whole flexible DC transmission system. Based on the theoretical analysis of the on-line monitoring methods for the capacitance, equivalent resistance and loss of DC supported capacitors, this paper proposes an on-line monitoring method for the capacitance of DC supported capacitors using the existing electrical monitoring of the bridge arm current, submodule voltage and switching state. This method does not need to change the existing valve control system hardware and communication architecture, and adds the capacity data analysis software and online capacity monitoring software on the basis of the original monitoring system to realize the real-time calculation of capacitor capacity and health status analysis. At present, the online capacity monitoring platform has been deployed and applied in the flexible converter station of Fukang Station in Zhangbei. Further analysis of capacitor short circuit or open circuit, excessive internal pressure, capacity attenuation and other fault conditions, proposed early warning, attempt to continue operation, attenuation reassessment, one pole warning, failure protection bypass and other multi-stage fault warning and failure protection protection strategies, effectively improve the operation reliability and safety of the converter valve. This paper provides a comprehensive technical scheme for the on-line monitoring and fault protection of the flexible DC converter valve DC support capacitor, and provides important guarantee for the safe and reliable operation of the core equipment of the HVDC transmission project.