The diesel spray characteristics at the upstream of a diesel oxidation catalyst （DOC） during active regeneration of a diesel particulate filter （DPF） and the diesel oxidation process in the DOC were investigated by means of integrated visualization experiment and numerical simulation. To the end， high speed photography device was used to capture diesel spray images， and the spray droplet sizes were measured with a phase Doppler particle analyzer （PDPA）. Based on the calibration of the diesel spray model and the DOC model， the diesel spray and oxidation process were simulated. Experimental results indicate that the higher the fuel injection rate， the greater the spray cone angle and penetration distance. The injection rate has a greater effect on spray droplet size distribution at the nozzle outlet. The SMD along the axial direction of spray varies with the number of large droplets. Simulation results show that the hydrocarbon distributions in the exhaust pipe and DOC inlet are determined by the diesel spray characteristics. The temperature distribution at DOC outlet has a close correlation with the hydrocarbon distribution at DOC inlet. The spray impingement and gravity force will cause hydrocarbon to accumulate at the bottom of exhaust pipe so that the mass concentration of hydrocarbon is close to 2%. While too much diesel fuel at the bottom of the pipeline will lead to lower temperature of the corresponding area at the bottom of the outlet section of the DOC， which is lower than 450 ℃. Increasing the length of exhaust pipeline can promote the mixing uniformity of the spray and gas， and alleviate the phenomenon of excessive concentration of diesel fuel at the bottom of the pipeline.