In order to ensure the work efficiency of High Efficiency Particulate Air Filter(HEPA),Dioctylphthalate method,which is widely used in the efficiency and leaking test of HEPA filtration system,is analyzed.The optical detect theory of Ddictylphthalate aerosol concentration is summarized.Based on Mie scattering theory,a mathmatical model is derived on relation between light-scattering intensity and concentration of aerosol when the special aerosol is generated.The optical measurement system based on the theory is designed and manufactured.The validity of the model is testified by the experiment.The experimental result shows that there is a good linear relation between light-scattering intensity and mass concentration of Dioctylphthalate aerosol.