In this study, a mathematical model based on the kinetic of Activated Sludge Model was established to describe the transformation of nitrogen compounds including nitrate and nitrite nitrogen, ammonia and ammonium nitrogen, soluble biodegradable organic nitrogen, and particulate biodegradable organic nitrogen in sewer. Then the effects of varied flow conditions and different initial dissolved oxygen (DO) concentrations on nitrification and denitrification were explored. The results showed that these four compounds were transformed during the transportation process. According to simulation, when DO varied and flow velocities were fixed, all nitrogen compounds varied slightly excepting nitrite and nitrate nitrogen. When initial DO was fixed, the different reaction time due to different flow velocity affected the nitrogen transformation significantly. Removal of nitrogen compounds was better when reaction time was greater.