The optical transmissivity is usually used to evaluate the properties of the aerosol.However,forward scattering and diffracting light by particles might reach the detector and would be incorrectly recorded,which would result in that the final optical density and experiment extinction cross section is erroneously low.The beam efficiency and effective extinction cross section are introduced to examine the error.The results indicate that the distance between the aerosol and the detector must be large enough in order to alleviate to error.