This paper applies the simititude theory, by means of the analysis of the discarging process of steam accumulator, and abtains the steam-moistness criterion correlation from mathematical equations to describe the process of carrying droplets in steam. The experiments are performed at the steam accumulator discharging test sret-up by using water as a working medium. The experimental data are correlated in the form of dimensionless equation, and the steam moistnees calculation is given, by means of regressive analysis.In industrial experiment of the practical steam accumulator, the data of steam-moistness measured in experiment and calculated by means of the calculation mentioned above are basically identical. This calculation provides a bassis for the vessel design and operation of steam accumulator.