Based on wave radiation transmission theory in random medium and Monte-Carlo method,the transmission characteristic of external mixture of aerosol which is composed of water soluble and soot with lognormal distribution is studied.The dependence of extinction coefficients of urban aerosol on incident wavelength is numerically simulated,which is compared with reference data.The accuracy of the discrete sampling Monte-Carlo simulation method is proved.The effect of incident wavelength,the cosine of incident angle,and optical thickness on the transmittance and reflectivity of laser in mixture aerosol is analyzed.Comparison is given between the external mixtures of aerosol with different mixing ratio.The numerical results show that the decrease of wavelength will lead to reduce transmission characteristic of external mixture aerosols and increase reflectivity.And the total transmission characteristic of external mixture aerosol is sensitive to its composition.