To address the failure risks induced by thermal accumulation in thyristors，which are core components of High-Voltage Direct Current transmission systems， an online monitoring system was designed. To resolve the issue of system-level monitoring interruptions caused by communication link anomalies under extreme operating conditions，this paper proposes an integrated waveguide chip structure applied to the monitoring system. First，a 4×16 multimode interference(MMI)coupler was designed and fabricated to achieve 16-channel high-density integration. Subsequently， a waveguide Bragg grating(WBG)sensor was developed to achieve high-precision temperature measurement. A multi-level redundant structure was then constructed by incorporating optical switches. The results demonstrate that the proposed scheme achieves real-time temperature monitoring of the thyristors with a measurement accuracy of ±0. 3℃ . Furthermore，when the primary channel fails，switching to the backup line can be completed within 20 ms. This effectively resolves the monitoring interruptions in traditional temperature monitoring systems caused by critical node failures，significantly enhancing system reliability and fault tolerance. The system has undergone engineering application testing at the Beijing Ultra-High Voltage Experimental Base.