A quantum phase transition arises from competition between different ground states and is typically accessed by varying a single physical parameter near absolute zero temperature. The quantum anomalous Hall (QAH) effect with high Chern number $C$ has recently been achieved in magnetic topological insulator (TI) multilayers. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI pentalayers by varying the thickness of the middle magnetic TI layer, designated as $m$ quintuple layers. Electrical transport measurements demonstrate a quantum phase transition between the $C=1$ and $C=2$ QAH states. For $m\le 1$ and $m\ge 2$, the sample exhibits the well-quantized $C=1$ and $C=2$ QAH states, respectively. For $1<m<2$, we observe a monotonic decrease in Hall resistance from $h/e^2$ to $h/2e^2$ with increasing $m$, accompanied by a peak in the longitudinal resistance. The quantum phase transition between the $C=1$ and $C=2$ QAH states is attributed to the weakening of the interlayer coupling between the top and bottom $C=1$ QAH layers. Our findings provide a scalable strategy for engineering QAH devices with a tunable Chern number. This approach enables precise control and enhanced functionality in chiral edge current based electronic devices.

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• Phase transitions
• Quantum anomalous Hall effect
• Topological insulators
• Transport phenomena
• Dilute magnetic topological insulators
• Molecular beam epitaxy