Abstract: With the main pollutant BOD₅ as an example and based on empirical analysis and calculation,the effect of great variation of water flow on the biodegradation rate of BOD₅( K₁)before and after the building of the Three-Gorges Reservoir was studied.An empirical equation of K₁ related with water velocity and depth was also developed.The results showed that after the building of the Three-Gorges Reservoir,the biodegradation rate of BOD₅ would obviously decrease with the increase of water level and decrease of water velocity.