To solve the problem of insufficient measured runoff data for new-built or to-be-built cascade reservoirs, five comparable daily runoff series of the Lhasa River were generated by using MODSIM-DSS for the post construction scenario. The indicators of hydrologic alteration and range of variability approach(IHA-RVA)was used to analyze the hydrological regime variation of the Lhasa River during the current situation and in the planning period. The results show that the cascade reservoirs have positive or negative accumulated impacts on a single indicator. The severest changing degree for a single indicator occurs in March and October, during which the changing degree of the monthly averaged discharges are both greater than 0. 9, being regarded as a severe change. The comprehensive hydrologic changing degree(CHCD)in the current situation is 0. 21, belonging to a slight change, in which the severe changed indicators account for 7. 8% of the total number. The CHCD in the planning year is 0. 41, belonging to a medium change, and the severe changed indicators account for 28. 1% of the total number. The CHCD increases as the increase of the total storage coefficient with a slow-fast-slow increasing rate.