Steam curtains are used for avoiding fire and explosion accidents in ethylene plants resulting from the ignitions of the released flammables by steam cracking furnaces. Currently, there are few references related to the methods for determing the design parameters of steam curtains. In this paper, the method for determining and adjusting the design parameters based on the release and dispersion scenarios of the flammables is put forward by studying the air entrainment and dilution effects of the steam jet from a steam curtain nozzle on the flammable cloud. It is shown that the method proposed by this paper connects the design paramaters of steam curtains with the selected release and dispersion scenarios, and it is conducive to proposing design paramters meeting the dilution performance required by the selected scenarios.