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In order to investigate the spray dispersion regulation of urea solution of diesel engine SCR (selective catalytic reaction) system, a urea spray model was built up to simulate urea injection and atomization process. The correctness of the model was verified by comparing the simulation and experimental results, the influences of exhaust gas temperature, exhaust gas flow, injection rate and injector position on dispersion of urea spray were analyzed. The results indicated that with the increase of exhaust temperature, particle density of urea solution on the inner wall of exhaust pipe was gradually reduced and the volume fraction of NH3 was increased, which tended to be stable at 400℃. But with the temperature further increased, NH3 oxidation was enhanced in the exhaust pipe, so its volume fraction was decreased. With the increase of exhaust gas flow rate, the packing position of the urea solution particles on the inner wall was moved backward obviously and the volume fraction of NH3 was decreased. Because with the increase of exhaust gas flow rate, the residence time of urea solution in the exhaust pipe was decreased, which was not benefit for the decomposition of urea. With the increase of injection rate of urea solution, the density of urea solution particles and the volume fraction of NH3 were increased gradually, the packing position of the urea solution particles on the inner wall was almost unchanged. With the increase of distance between urea injector and catalyst, the thermal decomposition and hydrolysis reaction of urea became more complete and the volume fraction of NH3 was increased accordingly.