Abstract: To make the experimental results reflect the actual situation of the prototype, the flow of pressurizer relief steam spraying was analyzed based on dimensional analysis method. The results show that the phenomena similarity between the model and the prototype should meet: Similar geometric structure; equal thermal parameters and same initial conditions, which can greatly simplify other similarity parameters; at the blow-down stage, keeping the steam mass flux density consistent; at the natural convection stage, the Fourier number and the Grashof number should be meet, while assessing relative uncertainty.