The development of cascade reservoirs significantly alters the water temperature of a river basin. However, our understanding of the distribution of water temperature in river basins under high-intensity hydropower development conditions remains limited. In this study, we analyzed historical data and monitored the present water temperature of the Wujiang River to assess the effects of cascade reservoir construction on water temperature and fish populations. The results revealed that the construction of cascade reservoirs weakened the continuity of water temperature in the basin. Various hydrological and meteorological factors influenced the water temperature of downstream dams. Among these factors, the upstream water temperature significantly impacted the downstream channel. There was no obvious pattern in the time needed to reach the thresholds of critical temperature and accumulated temperature for typical fish species ( Coreius heterodon) in the study area. However, the water temperature downstream of the Pingzhai Reservoir did not reach the threshold by the end of the monitoring period, while the water temperature downstream of other reservoirs allowed for normal Coreius Heterodon development prior to spawning. Furthermore, the construction of cascade reservoirs resulted in a significant reduction in the amplitude of water temperature at the downstream Wulong hydrological station, intensifying the water temperature delay in the Wujiang River. The findings of this study provide valuable insights into the cumulative effects on water temperature in the context of high-intensity hydropower development.