Ecological effects of the cascade hydropower dam construction on ecosystems have acquired much concern from scientists with the expanding of dam development scales in the Longitudinal Range-Gorge Region.In the Lancang River region and Yuanjiang River-Red River region of the Longi- tudinal Range-Gorge Region(LRGR),an index system has been therefore established to assess the current ecological vulnerability and to predict the likely future ecological vulnerability of these two river regions after the dam constructions,Eight cascade hydropower dam construction scenarios have been established in this paper for forecast.Abrupt change characteristics of the ecosystem have been con- sidered in the calculation.Economic benefits like current supply,navigation and tourisms of the dam construction have not been considered in the calculation,The results show that(1)the current eco- logical vulnerability values of Lancang River region and Yuanjiang River-Red River region are 0,34 and 0,43 respectively,which would rise to 0.90-0.91 and 0,86-0,89 respectively after the joint work of all the planned dams;(2)abrupt change would happen to both the Lancang River region and Yuanjiang River-Red River region in future;(3)interval construction with high human response is recommended for both the Lancang River region and Yuanjiang River-Red River region,after which their likely eco- logical vulnerability would rise to 0,56-0,62 and 0,42-0,43 respectively.These results provide a rea- sonable method and theoretical references to assess the ecological effects of dam construction,and also give prewarnings to other mainstreams of the Longitudinal Range-Gorge Region which are being planned to construct cascade hydropower dams. Ecological effects of the cascade hydropower dam construction on ecosystems have acquired much concern from scientists with the expanding of dam development scales in the Longitudinal Range-Gorge Region. The Lancang River region and Yuanjiang River-Red River region of the longi- tudinal Range- Gorge Region (LRGR), an index system has been therefore established to assess the current ecological vulnerability and to predict the likely future ecological vulnerability of these two river regions after the dam constructions, Eight cascade hydropower dam construction have been established in this paper for forecast.Abrupt change characteristics of the ecosystem have been con- sidered in the calculation .Economic benefits like current supply, navigation and tourisms of the dam construction have not been considered in the calculation, The results show that (1) the current eco- logical vulnerability values ​​of Lancang River region and Yuanjiang River-Red River regions are 0,34 and 0,43 respectiv ely, which would rise to 0.90-0.91 and 0,86-0,89 respectively after the joint work of all the planned dams; (2) abrupt change would happen to both the Lancang River region and Yuanjiang River-Red River region in future ; (3) interval construction with high human response is recommended for both the Lancang River region and Yuanjiang River-Red River region, after which they likely eco-logical will would rise to 0, 56-0, 62 and 0, 42-0 , 43 respectively.These results provide a rea- sonable method and theoretical references to assess the ecological effects of dam construction, and also give prewarnings to other majors of the Longitudinal Range-Gorge Region which are being planned to construct cascade hydropower dams.