In the case of desulphurization plant design, we have to design ones which meet Such a various requirement as using low quality's fuel or operating with high desulphurization rate. In order to design them, first of all, we need to make a simulation model which can be accurately simulated for an actual plant performance. In the next place, we need to reflect a simulation result to an actual design rapidly. In this report, we selected the limestone-gypsum process, which were adopted at most places in large-sized desulphurization processes for power plants, and the spray-type absorber as a desulphurization reactor. We made a simulation model by which can be calculated loci of many drops and SO2 absorbed reaction rate. We predicted desulphurization rate toward sonic unknown flue gas velocities by using the simulation model. As the result, we showed an optimized design value, which made spray abosorber's volume small, as well as made it possible to operate with a required desulphurization rate, by increasing a flue gas velocity and lowering height of spray absorber. The simulation result was reflected to an actual plant design, and it was proved to fulfill a desired desulphurization rate, as the result of construction and operation of the plant which were designed at the condition of high flue gas velocity.