Abstract: Hydropower development in the Jinsha River Basin has altered the thermal regime, profoundly affecting the habitat of aquatic organisms. To investigate the water temperature response patterns of the Panzhihua reach of the Jinsha River under cascade development, water temperature data from the Panzhihua and Sanduizi hydrological stations were analyzed. Mann-Kendall trend test, piecewise linear regression, and regression discontinuity design were used to examine interannual, intra-annual, and longitudinal water temperature trends, as well as the impact of the Guanyinyan Reservoir and other cascaded reservoirs impoundment (hereinafter referred to as "Guanyinyan Impoundment") on the river’s thermal regime. The results showed a significant increase in the average annual water temperature in the Panzhihua section over the past 50 years. After 2012, the longitudinal water temperature trend changed significantly, with the temperature in the river section above the Yalong River confluence being significantly higher than that below. The impoundment of the Guanyinyan Reservoir significantly impacted the downstream water temperature, with intra-annual distribution tending to flatten, leading to a notable “warming effect” in autumn and winter and a “cooling effect” in spring and summer. The degree of water temperature homogenization in the river section below the confluence of the Yalong River was greater than that in the section above the confluence. This study reveals the impact of the Guanyinyan Reservoir impoundment on the thermal regime of the Panzhihua section of the Jinsha River, providing a scientific basis for ecological dispatching and the protection of fish breeding and habitat environments.