A semi-empirical theoretical method for water droplet size fast calculation is presented to resolve the problem confronted with the measurement of wet steam scattering light distribution in steam turbine. Phase function for different droplet size is numerically calculated based on the Mie scattering theory, and by Gaussian fitting of the phase function curves in the forward small angles, we get an analytic approximation resolution for Mie scattering function. At last, a semi-empirical function for light scattering ratio with the droplet parameters is obtained by further curve fitting. Light scattering experiment for water droplets size measurement is carried out in a simulated steam turbine under various work conditions. Scattering light intensity distribution is received using a CCD camera, and the droplet radium is real-time inversed by means of the semi-empirical function. Because of the restriction of the range of the droplet size distribution and the approximation of the theory, the improved model can well resolve the problem of wet steam droplet parameters online inversion.