Abstract: On the basis of studying the transformation principle of nitrogen in the middle-stream of the Songhua River the authors found that the change of real measured concentration of ammonianitrogen did not conform to the graded dynamic model adopted extensively under low water temperature(especially in freezing phase).The real concentration of ammonianitrogen is mainly affected by the speed of transforming organonitrogen into ammonianitrogen and the speed attenuation of ammonianitrogen.And the authors advance that the water quality of model of the Songhua River should be set up with the theory of chemical dynamic successive reaction and estimate the environmental capacity of ammonianitrogen pollution in the Songhua River.