In order to improve the A-P value method,a rational method for distributing permissible sulfur dioxide(SO_2) emission to urban area sources is put forwarded.The permissible SO_2 emission calculated by the A-value method was added to SSIM(Source Strength Inversed Model) as a restriction condition to adjust the sharing ratio of different pollution sources.When the calculated sum of permissible area source emission in each grid for urban control areas is equal to the restricted value,the model output is the distributing result for permissible emission in urban control areas.It was used to calculate the grids' permissible SO_2 emission in Shijiazhuang,and the effectiveness was verified by using ISC3LT.The results show that with the method,SO_2 concentrations can be controlled within the National Standards on the grids of the control areas(urban areas).