In-situ measurements of particle number size distribution and optical parameters (scattering and absorption coefficients), covering the particle size range of 3 nm-2.5 μm, were conducted at Mt.Tai, a mountain top station in Central East China in spring time 2011.During the whole campaign, three episodes with significant influence by the Asia dust outflow were captured.The mean mass concentration of PM2.5 during the dust episode was 63 μg m-3, nearly 2 times as the value for the non-dust period, 33 μg m-3.But the difference in PM1 mass concentration is not significant due to a high proportion (about 50%) of coarse particle in PM2.5 mass on dust days.Although it showed relatively low number concentration of the incoming dust particles, they changed the optical properties and particle volume size distribution significantly.Compared with the non-dust period, the absorption coefficient was increased by 100% during dust period.The angular truncation corrected Nephelometer measurement showed that scattering coefficient was nearly 30% lower on dust period.The (A)ngstr(o)m exponent and single scattering albedo are also altered by the dust particles.The spherical Mie model was applied to simulate the optical properties with reasonable assumptions and was validated through closure analysis for an intensive dust case.Results of calculation suggested that the dust particles particle contributed nearly 20-60% to the aerosol optical properties during the dust outbreak period, with the mean contribution of 30% to scattering and backscattering coefficient and 20% to absorption coefficient,although the number concentration is only several tens per cubic meter.