Following our simulation results in paper (Zhao et al., 2020). In this study we continue to analyze the diffusion mechanism of ultrafine particles and the particle coagulation phenomenon with the size range of 26~287 nm exhausted from the vehicles during the process of passing through a 100 m long tunnel using the Realizable k-ε model and the dynamic grid technique. In this paper, a three-dimensional model consisting of a 100 m highway tunnel and four sideby-side gasoline vehicles (L×W×H = 4.5 m×1.8 m×1.5 m) were established in the STAR-CCM+ computational fluid dynamics software. The gasoline vehicles travelled simultaneously under the different situations of three driving speeds of 60 km h -1 , 40 km h -1 and 20 km h -1 during the simulation. Through data analysis and research, it is found that the coagulation process of particles is very complicated, especially at low speeds. When the vehicle speed is 20 km h -1 , the variation of particle concentration at the vehicle wake near the tailpipe (at the vertical plane located 0.1 m behind the exhaust pipe) will cause a large error if the coagulation action is not taken into account. The relative error of the average particle concentration at 0.5 s of the vertical section 0.1m away from the exhaust pipe is as high as 193.51%. The relative error in the whole tunnel is only 2.82%, less than 5%, thus the influence of coagulation can be ignored for the whole tunnel. This study clarified the importance of coagulation in different areas and its influence on the diffusion of particulate matter, which is conducive to further analysis of the diffusion characteristics of particulate matter and can appreciably reduce the pollution degree in the tunnel by changing the coagulation efficiency of particulate matter in the future.