Abstract: A composite membrane bioreactor (CMBR) integrating the membrane separation technology with the immobilized cells technique was investigated for groundwater denitrification. The optimal denitrification conditions were obtained by orthogonal and single factor experiments, respectively. The results showed that the optimum conditions of immobilized cells preparation were 4 mm thickness, and embedding denitrifying bacteria in PVA solution (w=15%) was 20 g/L. The optimum carbon to nitrogen ratio, temperature and pH were 6∶1, 30 °C and 7.0-8.0， respectively. And ethanol was a proper carbon source. The denitrification rate of CMBR was proportional to the initial concentration of NO3-N. The water quality indexes of nitrate nitrogen, nitrite nitrogen and chemical oxygen demand (CODMn) in treatment groundwater under the optimum conditions can meet the requirements of current national standards for drinking water.