The sensitivity of a laser optical particle counter (L-OPC) measuring to the refractive index of particles is analysed, and the dependence of response on ni, the imaginary part of the refractive index is discused. Sensitivity function fi is defined. A scattering angle less than 20° is chosen and another is between 40° and 60° according to analysis of theory. For example, scattering angles system ψ=9°, β=5°, λ=0.65 μm and ψ=50°, β=20°,λ=0.65 μm, the one is slightly not effacted by ni, the other is greatly. By useing different sensitivity of different scattering angles to ni, the value of ni can be deduced from the measurement by a laser optical particle counter of double scattering system. The simulation method will be used to deduce ni.