The DC Link capacitors in the flexible DC transmission system undertake tasks such as energy storage, voltage fluctuation mitigation, and harmonic filtering, playing a crucial role in maintaining system power balance and enhancing power quality. However, capacitors are susceptible to factors such as current shocks, temperature variations, and harmonic pollution during operation, which can compromise their reliability and lifespan. Therefore, real-time monitoring and condition assessment of DC Link capacitors are of utmost importance. Currently, monitoring technologies for capacitor reliability indicators are mainly categorized into model-based and sensor-based approaches. Model-based methods estimate capacitor capacitance and internal temperature through electrical and thermal models, but they rely on accurate modeling and have poor adaptability to varying operating conditions. Sensor-based monitoring techniques can directly acquire capacitor status information, but they are often invasive, increasing system complexity and potentially interfering with capacitor operation. Additionally, existing research primarily focuses on monitoring the overall DC bus of the system, whereas individual capacitors exhibit significant reliability differences due to consistency issues and variations in electrothermal stress caused by parameter design. At present, there is a lack of online monitoring methods specifically for individual capacitors. To address these issues, this paper proposes an innovative capacitor state sensing unit that integrates current and temperature data monitoring with wireless data transmission capabilities, enabling real-time monitoring and early warning of capacitor status. This method employs a non-invasive online electrical monitoring circuit, utilizes a thermal network model supplemented by temperature correction from three test points on the capacitor casing to monitor the internal temperature rise distribution of the capacitor, and leverages wireless communication technology for remote data transmission, enhancing system flexibility and scalability.