Graded crushed stone is widely used as the base filler of expressway pavement. As an important means to determine the strength of subgrade soil and pavement materials, California Bearing Ratio (CBR) test can reflect the actual bearing capacity of the subgrade or pavement structure layer. Based on the particle flow software PFC3D, the influence of mesoscopic parameters on the CBR test is studied. With the help of the force chain model in PFC3D, the internal force evolution process of the sample is observed, and the influence of particle microscopic parameters such as particle stiffness ratio kn/ks, friction coefficient μ, normal bond strength Tσ and tangential bond strength Sσ on the test results is discussed. The results show that kn/ks has no significant effect on the CBR value, μ and Tσ both have a linear positive correlation with the CBR value. Then, the influence of normal and tangential bond strength on the CBR test results is compared using the bond strength ratio Tσ/Sσ. It is found that Sσ has a significant impact on the CBR value, it mainly determines the tangential plasticity of the contact key, whereas Tσ mainly controls its surface normal adhesion.