Abstract: In order to solve the problem confronted with the online steam humidity measurement of turbine flow pass, a semi-empirical formula of phase function computing method was proposed to improve the inversion algorithm on scattering light distribution. Phase function of double Gaussian fitting by using MATLAB with cftool was adopted in this algorithm. The fitting result shows that the relative error is in small, and the approximate analytic solution of the Mie scattering theory could be got by the fitting function, which means that a rapid inversion from the scattering light intensity distribution to water quality radius. According to the results, with the restriction of the range of the droplet size, the approximation of the theory between the droplet size and scattering ratio was got. Contrast experiment showed the measured scattering ratio was similar with the simulated scattering ratio. Based on the semi-empirical model, the water quality of middle radius was 1.1μm.The fast inversion algorithm can well resolve the online measurement of steam humidity.