Abstract: A fertilizer that served as an important agricultural input, become rank first in the world in terms of Chinese production and utilization, further ensuring national food security and agricultural productivity. However, there are various effects of the shape characteristics of fertilizers on the appearance quality, strength, fluidity and the efficiency of mechanical fertilization in modern mechanized agriculture. If the sphericity of particles in a fertilizer is high, the strength of the fertilizer will be high, indicating hardly being deformed and/or broken. If the roundness of particles in a fertilizer is high, the porosity of the fertilizer will be large, indicating the better heat dissipation and flow effect. There are significant influences of particle shape and pore structure on the diffusion of salt ions in the fertilizer, while in turn the agglomeration of the fertilizer can affect the pore structure that caused by the accumulation of the fertilizer. Therefore, the effects of different shape characteristics on the fertilizer sphericity were studied by using single-factor and central combination tests, in order to clarify the relationship between various shape features and mechanical properties in the fertilizer. Firstly, the length, width, thickness, equiaxed rate, flake rate, roundness and sphericity of fertilizers were measured by the shape analyzer for agricultural materials. Secondly, the correlation coefficient between different shape characteristics and the fertilizer sphericity was determined by the single-factor variance test. Finally, taking the equiaxed rate, flake rate, roundness of fertilizers as the main influencing factors, the mathematical model of the fertilizer sphericity was established by Box-Behnken central combination test, and then the relationship between these factors and fertilizer sphericity was analyzed by the approaches of the regression statistical variance, response surface and isoline. The simulated results showed that a significant multiple regression equation could be fitted between the fertilizer sphericity and the equiaxed rate, flake rate, and roundness. The influence degree of each factor on the fertilizer sphericity from high to low was in the order of the equiaxed rate, flake rate, and roundness. When the equiaxed rate was in the range of 0.98-1.00, the flake rate was in the range of 0.92-0.95, and the roundness was in the range of 0.85-0.88, while the fertilizer sphericity was the highest, reaching over 92.9%. In the same batch of the compound fertilizer, 50 particles were randomly selected as verification samples, and the measured sphericity of fertilizer was obtained through various experiments. The predicted sphericity of fertilizer was achieved by the sphericity regression equations, while the accuracy of the sphericity model was evaluated by the error rate. The test results show that the maximum positive and negative error rate between the measured- and predicted sphericity of fertilizer was within 9%, indicating that the test results were consistent with the simulated values from the response surface analysis, as well verifying the proposed mathematical model. Therefore, a novel method for the indirect measurement of the fertilizer sphericity was proposed, which can provide significantly theoretical fundamentals for the production and quality inspection of fertilizers.