The invention discloses a design method for a semi-dry method flue gas desulphurization system process. The design method comprises the following steps: (1) establishing a mathematical model for parametric computation of a tower body of a rotary spray drying tower and a bag-type dust remover in a semi-dry method flue gas desulphurization system; (2) measuring concentration information of SO2 in flue gas, flue gas flow rate, flue gas flow velocity and flue gas temperature information through a measuring instrument; (3) combining measured information and mathematical model computation to obtain the design parameters of the tower body and the bag-type dust remover. The mathematical model is reasonable in design, can be used for conducting accurate computation to obtain system process design values of tower diameter and tower height of the rotary spray drying tower, the filtration area of the bag-type dust remover, and the like, and provides theoretical computation bases and references for system design. In addition, the design method can improve the desulphurization efficiency of small and medium sized boilers; through the adoption of the bag-type dust remover, the desulphurization efficiency can be improved, solid particles and fly ash components in flue gas can be intercepted, and the dust pollution can be reduced.