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We employ the two-band density-matrix equations and time-dependent density functional theory to calculate high-order harmonic generation (HOHG) in graphene under intense mid-infrared laser irradiation. Our investigation reveals a distinct harmonic enhancement structure (HES) within a specific energy range of the HOHG spectrum. In this regime, we find the convergence of multiple interband electron-hole recombination trajectories, leading to the divergence of the Hessian matrix term of the semiclassical action. This convergence exhibits characteristics akin to the focusing behavior of light rays, commonly known as caustic effects. In contrast to atom situation, where caustic effects are confined to a narrow energy range around the HOHG cut-off, and the enhancement due to caustic trajectory convergence is less apparent, the two-dimensional nature of graphene results in a broad energy region for HOHG enhancement. In fact, the caustic trajectories can even dominate the entire process of interband harmonic generation. The magnitude of enhancement becomes significant and can be estimated to be on the order of $\sim N^{2/3}$, with $N$ representing the harmonic order, according to the catastrophe theory. These mechanisms have broad applicability and hold significant implications for other two-dimensional materials, as well as bulk materials. Our study provides crucial insights into the understanding of HOHG phenomena in diverse material systems.