To face challenges of unstable discharge for feed structure with equal diameter variable pitch screw, a theoretical analysis for variable pitch screw capacity was presented under some simplification assumption, which was compared with the equal pitch one, and resulted in a flow calculation formula by reasonable mathematical derivation. Furthermore, design of variable pitch screw was optimized by using response surface method which a flow rate was set as the response value in accordance with the above calculation formula. The BoxBehnken test analysis was performed with the help of DesignExpert software, turning out that the optimal pitch combination for the smallest target flow error was S1=0.35D, S2=0.60D, S3=0.85D, and S4=D. After all, movement simulation for millet as transport medium were performed in the screw with obtained optimum design, which the 3D model was built with SolidWorks software and the EDEM software was imported for discrete element analysis. After calibration, the result indicated that the simulative mass flow was 0.321kg/s, compared with calculation flow value of 0.319kg/s and the difference rate of 0.627%, relative error of which was measured to the actual required flow was 3.7%. The reliability of the flow formula was verified while the feasibility of the response surface method to design the screw structure was proven, which provided a reference for the design of the quantitative variable pitch screw. 