Bio-butanol has been considered to be an important alternative fuel to reduce soot emissions from gasoline engines, and it has the potential to overcome the disadvantages brought by low-carbon alcohols. In this study, the effect of n-butanol addition on soot formation in gasoline laminar diffusion flame is investigated respectively under the same fuel flow and the same carbon flow. The soot volume fraction distributions in the flames are measured using the two-color laser induced incandescence (TC-LII) technique. Furthermore, the But-TRF-PAH mechanism is constructed and used to simulate the formation and oxidation of soot precursors in the CHEMKIN zero-dimensional constant pressure reactor. The experimental results showed that n-Butanol addition can significantly reduce the soot formation, and the measured peak and average soot volume fraction both decrease linearly with the increasing n-butanol ratio. For a given n-butanol ratio, the reduction of soot under the same carbon flow (SCF) condition is only slightly smaller than that under the same fuel flow (SFF) condition, which indicates that the decreasing carbon mass flow due to n-butanol addition has little contribution to soot reduction. The simulated results showed that benzene is mainly produced via C6H5CH3 + H = A1+CH3, and is mainly consumed via A1 + OH = A1- + H2O and A1 + CH3 = A1- + CH4. The production pathways of PAH is consistent for the toluene reference fuels (TRF) and n-butanol/TRF blends. The ratio of benzene accounts for toluene consumption is essentially constant, about 1/4. Thus, the n-butanol addition reducing the soot is mainly due to the dilution of the amount of aromatics (toluene in TRF) in gasoline. Furthermore, the amount of OH and HO2 radicals increases with increasing n-butanol ratio, which have an important indirect effect on the soot precursors formation.