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The geometric model of a 50-inch agricultural axial fan was established by a 3D laser scanner. The experimentally verified numerical model was used to study the influence of length of shroud of agricultural axial fan on the internal and external characteristics of fan. Keeping the inlet position of the shroud, the rounded corner radius and the diffusion angle were unchanged, and defining the ratio of length of shroud to axial width of tip as K. The value of K was chosen as 0.50, 0.78, 0.90, 1.0, 1.1, 1.2, 1.5 and 2.0. Totally seven different imported static pressure operating points were chosen for numerical simulation. The results showed that during the process of increasing the K from 0.50 to 1.0, the air volume and energy efficiency ratio of fan were obviously improved. When K was from 1.0 to 1.1, the fan air volume and energy efficiency ratio were the highest, which was about 10% higher than those of the prototype fan with K=0.78. When the K was from 1.1 to 2.0, the air volume was slightly decreased. The energy efficiency ratio was decreased significantly. Using the Q criterion to identify the vortex structure in the tip and outer frame regions, it was found that with the growth of K, the tip leakage vortex underwent three processes of division, attenuation and redevelopment. When K was 1.0 it can effectively inhibit the development of tip leakage vortex and minimize the strength of tip vortex. The lengthening of the shroud can significantly reduce the outer frame vortex.