In order to explore the effects of effective microorganisms(EM)on the improvement of water quality of crab ponds and survival rate of crabs, an experimental study on EM application was conducted in the Crab Cultural Park in the Gaochun District of Nanjing City. The results show that applying EM to Eriocheir sinensis culture can effectively decrease the concentration of pollutants in the water and increase the survival rate of crabs, but overdoses of EM should be avoided. With an EM concentration of 1. 5 mg/L in the crab pond, the concentrations of NH⁺₄-N, NO^-₂-N, COD, and TP decreased by 12. 7% to 29. 2%, and the survival rate of crabs increased by 2. 4%, compared with the control group. However, when the concentration of EM in the crab pond increased to 6. 0 mg/L, the concentrations of NH⁺₄-N, COD, and TP decreased by 1. 1%, 14. 4%, and 0. 03%, respectively, while the concentration of NO^-₂-N and the survival rate of crabs increased by 15. 6% and 1. 0%, respectively. A high EM dosage of 6. 0 mg/L resulted in poorer performance in improving the water quality and survival rate than a small EM dosage of 1. 5 mg/L, and even caused accumulation of NO^-₂-N.