To improve the growth and efficiency of aerobic denitrifying bacteria, by adding different dose of magnetic powder (within 05-30 g／L, 05 g／L for a gradient) and after 24 h, the growth absorbency, nitrate removal rate and dehydrogenase activity were determinated. Results show that the growth increases, and the nitrogen removal efficiency and dehydrogenase activity of T13 first increase and then decrease with increasing of magnetic powder dosage. The magnetic powder promotes the aerobic denitrifying bacteria growth and efficiency, and 20 g／L of magnetic powder is the optimal dosing quantity, under which the dehydrogenase activity and denitration nitrogen removal of T13 go to the maximum 739 μg／(mL·h) and 100％ respectively.