This paper aims to propose a function fitting method to solve the problem that the variable pitch spiral structure is mainly based on empirical design and there is a lack of system design methods. First, the initial fitting of the variable-distance spiral expansion line was performed by the quadratic function y=Ax2+Bx. Considering the uniform variation of the pitch-changing spiral of the feed section, the second function was performed by the linear function y=Ax+B. Based on the result of leaner fitting, discrete element of equidistant and variable pitch spirals was simulated with millet material with a particle size of 1.5 mm. The variable pitch design of the feed section was obtained as follows: S1=0.4D，S2=0.56D，S3=0.72D, and S4=0.8D. Compared with the equal pitch, at the same speed and blade outer diameter, the feeding section of the variable pitch spiral structure was relatively uniform, the conveying was relatively stable, and the relative error between the mass flow and the actual flow was 1.8%. The simulation results verify the rationality of the design of the variable pitch spiral structure, and have certain reference value for the study of the variable pitch spiral structure.