In this paper,based on Mie scattering theory and Lambert-beer law,a steam humidity measurement system is established by using CCD camera to receive the scattered light intensity of particles of wet steam droplets.The theoretical calculation models of this system are respectively using LN distribution and (R-R) distribution to describe the particle size distribution of water mist in the simulator cylinder steam.The scattering light intensity is calculated by simulation,and the simulation results are compared with the experimental data.The results show that the theoretical calculation model based on lognormal distribution (LN) is more suitable for steam humidity measurement by backscattering method.