In constructed wetlands, the main occurrence formation of nitrogen includes organic nitrogen and inorganic nitrogen, and the inorganic nitrogen includes ammonia nitrogen and nitrate-nitrogen. The different formation of nitrogen is transformed reciprocally through some physical, chemical and biological process, such as mineralization, ammonia and nitrate-nitrogen assimilation, nitrification, denitrification, adsorption and exchange of ammonia-nitrogen, plant uptake, plant decaying and origanic-nitrogen sedimentation. Furthermore, in constructed wetlands used by wastewater treatment, the nitrogen solutes are transported by saturated Darcy flow and dispersion flow in porous medium. According to transformation mechanisms and solute transportation of nitrogen in constructed wetlands, the theory model of nitrogen transformation and transportation is based on solute transportation equations, and mechanism courses of nitrogen transformation are included in the basic transportation equations as source items and sink items. The state variables of mathematical model include organic, ammonia, nitrate, and plant uptake-nitrogen in constructed wetlands. Furthermore, the reference value of model parameters is given according to literature, and solution approach of how to use MATLAB to solve the model is given. This model can be used to estimate quantificationally nitrogen removal efficiency by different nitrogen transformation process and different influence factors, which can give quantificational parameter for the design of constructed wetlands, such as HLR(Hydraulic Load Ratio), HRT(Hydraulic Retention Time), Length of constructed wetlands, etc.