Based on a prototype installation, the mixing time and the fluid flow pattern of CAS (Composition Adjustment by Sealed argon bubbling) were studied by a water model analog simulation. The decisive criterion for this simulation was formulated as a dimensionless number Qμ/(D2σ) which can satisfactorily keep the physical similarity between model and prototype. The critical bottom blowing gas flowrate due to the existence of a slag baffle was defined and used to evaluate the reliability of the simulation. It was found from the investigation that the fluid flow behaviour of CAS is significantly different from that of the conventional ladle bottom-blow stirring. Comments on the design and operation of CAS were proposed. The optimization of CAS was recommended to be conducting the operation with both eccentric bottom blowing and eccentric slag baffle positioning by using ladles of large height/diameter ratio