The size multiplier,the coefficient of different size particles in the road dust emission factor model, is one of the important parameters of road dust emission inventory, which directly affects its uncertainty. This study collected 88samples on 11roads in Tianjin City with quadrat sampling method in spring of 2015. On the basis of portable Grimm1.109 aerosol particle spectrometer and resuspender experiment, PM_2.5 size multipliers of the road dust (K_2.5) were obtained by the formula. And then the K_2.5 distribution features were analyzed. The results were as follows: the K_2.5medians of non-motorized vehicle lanes ranged from 0.053g/VKT to 0.088g/VKT, and the K_2.5 medians of motorized vehicle lanes were 0.047~0.087g/VKT; the K_2.5 median values of non-motorized vehicle lanes in major arterial, minor arterial, branch road and express way were greater than those of the slow lanes, but the K_2.5 value of the slow-vehicle lanes in outer ring was a little larger than that of the non-motorized vehicle lanes; the sequential orders of K_2.5 distribution in different types of roads were outer ring > minor arterial > major arterial >branch road > express way in the case of motorized vehicle lanes, and minor arterial > branch road > major arterial > outer ring > express way in the case of non-motorized vehicle lanes; the K_2.5 median values of the same type lanes in two roadsides were different, but there was no statistically significant differences in-between them.