In the spawning area of Acipenser sinensis of downstream waters of the Gezhouba Dam, the hydrological conditions in spawning seasons (October-November) before (from 1983 to 2002) and after (from 2003 to 2005) the operation of the Three Gorges Project (TGP) were analyzed. It showed that the hydrological conditions during the spawning seasons were changed obviously since the operation of Three Gorges Reservoir (TGR) in the year of 2003, and that average water temperature in the spawning season was increased by 1.48℃ ( 8.2% ), and the sediment content was decreased by 0.36 kg/m3 ( 91.8% ). A suitability model based on hydrological conditions was proposed to evaluate the suitability of daily hydrological condition for spawning during the spawning seasons before (from 1983 to 1986) and after (from 2003 to 2005)the operation of the TGP. The average suitability model showed that spawning suitability for spawning increased at first and then decreased in the whole spawning season. However, the spawning suitability was comparatively high in the whole spawning season. The spawning activities of Acipenser sinensis don’t always take place in the best situation time, i.e. the suitability of hydrological condition is the highest. That is to say, the hydrological condition is not the only factor to determine the spawning time. The variation trend of sediment content before spawning used to decrease in steady before the TGP operation, however, the trend increased in steady after the reservoir operation. It means that the sharply decrease of silt content may bring on adverse effects on the natural reproduction of A. sinensis. In overview, the occurrence time of natural spawning activities of A. sinensis should be the integrated results of hydrological conditions, scientific-study capturing activities, differences of spawning cohorts and some other possible factors