The quantum anomalous Hall effect (QAHE), achieved through in-plane magnetization or characterized by a high Chern number, provides an opportunity to examine unconventional quantum states. In addition, it offers theoretical guidance for the development of spintronic devices with low energy consumption. Based on crystal field theory, we propose a general design principle for constructing in-plane-magnetized QAHE systems, and systematically analyze the mechanism by which chemical functionalization of two-dimensional (2D) materials induces this effect. Using first-principles calculations, we further predict that a MnSeCl monolayer can intrinsically host in-plane-magnetized QAHE, whose nonzero Berry curvature and chiral edge states unambiguously characterize its nontrivial topological nature. Moreover, our study reveals that tuning the in-plane magnetization direction enables Chern number switching under threefold rotational symmetry ($C=\pm 1$). When the magnetization is rotated out of plane, the system transitions into a high-Chern-number ($C=\pm 3$) QAH state. Wannier tight-binding calculations confirm the essential results and clarify their physical origin. In addition, chemical functionalization provides an effective route to realize in-plane QAHE in 2D systems and enables high-temperature platforms with tunable Chern numbers for low-dissipation spintronic applications.

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• Edge states
• First-principles calculations
• Magnetic anisotropy
• Quantum anomalous Hall effect
• Spin-orbit coupling
• Topological insulators
• Topological phase transition
• 2-dimensional systems
• Chern insulators
• Dirac semimetal
• Electronic structure