The collector is one of the main components of the axial flow fan, which has a great influence on the fan performance. However, the problems of unreasonable structure design, high energy consumption and air volume loss are common in the collectors of agricultural axial flow fans. Effects of the inlet length (L), fillet radius (R), and the outlet diameter (D) of the collector on the fan performance and flow field were studied, taking a typical 24 inch negative pressure FRP axial flow fan as the research object, using wind tunnel tests and CFD simulations. Besides, single factor analysis and response surface analysis were used in this process. The results showed that the suitable ranges of the dimensionless parameters of the collector were 1.00≤EL≤1.46, 2.95≤ER≤3.22, 0.9898≤ED≤0.9971. The effects of each influencing factor on the performance of the fan were obtained through the response surface model. When L=149.27mm, R=321.68mm, and D=678.00mm, the fan performance can be achieved better, the optimized Q was increased by 7.58% and N was increased by 8.07% compared with the prototype fan, the Q=10.06145m3/h, N=20.24m3/(h·W). The numerical simulation indicated that the optimized Q was increased by 5.86% and N was increased by 6.79% compared with the prototype fan, the Q=9900.54m3/h, N=20.03m3/(h·W). The optimized collector can significantly increase the positive vortex value in agricultural axial fan shroud，the maximum positive vortex of the leaf top clearance was 2197.21s-1, and its distribution area was increased in agricultural axial fan hub and shroud. The research result can provide a parameter optimization method to improve the airflow and energy efficiency ratio for agricultural axial fan collectors.