<p>Hydrothermal condition is one of the most important factors influencing fish reproduction. Successful reproduction requires that both the critical temperature (CT) and accumulated temperature (AT) thresholds are reached at appropriate times. This study addressed two key questions: (a) What mechanisms enable climate change and reservoir impoundment to influence the attainment of CT and AT thresholds and their match relationship by fish? (b) How can the matching theory between CT and AT thresholds be applied in practical assessments? To address these questions, a conceptual framework was developed. The framework was applied to the upper Yangtze River to evaluate the effects of climate change and the Xiangjiaba (XJB) reservoir on the reproductive hydrothermal conditions of the Yangtze sturgeon (YS). Results indicated that while XJB reservoir impoundment was the primary driver of changes in the river’s natural hydrothermal regime, climate change made a substantial contribution to the interaction effect. The match degree of alignment between the timings of CT and AT thresholds can effectively quantify the adverse effects of climate change on fish reproduction. Climate warming had reduced the suitable ecological niche in historical reproductive areas by 50%. Reservoir impoundment restored temporal alignment between thresholds in reproductive section, but further delayed start reproductive time, exacerbating conflicts between the long reproductive period and narrow reproductive window. The ecological niche suitable for YS reproduction quantified by match degree was narrower than that estimated using CT or AT alone. The methodology developed in the study provided scientific basis for optimizing reservoir ecological regulation and conserving rare fish species under climate change.</p>