Abstract: To address the lack of systematic comparison and quantitative selection criteria regarding the failure behavior of hybrid redundancy configurations for temperature relays in spacecraft thermal control electric heating circuits, this study performs a reliability analysis and failure mode comparison of two series-parallel redundant configurations. Both architectures achieve extremely high reliability (≥0.999 990) but exhibit distinct fault-tolerance characteristics. The intra-group series and inter-group parallel configuration effectively suppresses unintended energization (false ON) caused by contact sticking (17 cases), yet is more susceptible to unintended open-circuit (false OFF) (25 cases). Conversely, the intra-group parallel and inter-group series configuration exhibits strong tolerance to unintended shutdown due to open-circuit failures (17 cases), but provides weaker suppression of unintended energization (25 cases). These differences arise from the distinct logic and failure propagation paths of the two configurations. Based on failure probabilities, system safety priorities, and resource constraints, targeted engineering recommendations are proposed, providing theoretical support and decision-making references for redundancy design in spacecraft thermal control electric heating circuits.