Magnetic matrix is a key component of high gradient magnetic separation (HGMS) system. Special shape matrices have brought great attentions from researchers whereas only elliptic and circular matrices have been properly modeled and comprehensively investigated. Our previous study showed that diamond matrices present good magnetic characteristics in axial HGMS and are worthy of being comprehensively studied. In this paper, a 2D dynamic simulation model was developed to representatively describe particle capture for the 3D axial HGMS. Quantitative analysis on particle capture cross section (PCCS) revealed the matching relation between aspect ratio and magnetic induction for maximum particle capture by diamond matrix. It is found that the optimal aspect ratio decreases as magnetic induction increases, and is independent on particle size (in the size range of 1–30 μm). The advantage of favorable aspect ratio is more prominent for smaller particles under lower magnetic induction. Comparative study reveals that effect of aspect ratio on particle capture performance of diamond and elliptic matrices are quite similar, and their optimal aspect ratios are nearly identical under specific induction. Diamond matrix exhibits a slight superiority over elliptic matrix in most cases, making it a kind of promising matrix in axial HGMS. An empirical equation has been proposed to determine the optimal aspect ratio, and it is estimated that the optimal aspect ratio of elliptic and diamond matrices are about 0.75‧Ms/B0.