In order to analyze the characteristic of particle coagulation from vehicle exhaust under constrained dispersion conditions and the effect of coagulation on dispersion in an underground parking lot, the study numerically simulated the particle concentration fields under steady-state constrained dispersion conditions using Realizable k-ε model and field functions based on the theories of particle dispersion and coagulation, and then analyzed the characteristics of particle decrease half-time due to coagulation and variation of particle size distribution. The simulation results demonstrated that particle decrease half-time was shortest near the tailpipe exhaust. In the area lower than 2 m in vertical direction in an underground parking lot, when idling cars were not more than 2, half-time decreased linearly with the increase of distance above the ground; when idling cars were not less than 4, half-time decreased logarithmically with the increase of distance above the ground. The effect of coagulation on variation of particle number concentration in the middle and large size categories was more obvious than that in the small size categories. Particle size changed from small to large due to particle coagulation. The uneven distribution of particle size was more obvious as the number of idling cars increased.