Based on the macro-policy context of the construction of a unified national market, this paper takes regional engineering supervision enterprises as the research object, aiming to address the strategic decision-making challenges in their cross-regional market expansion. By integrating institutional theory, the resource-based view, and market entry mode theory, the study constructs a market boundary expansion decision-making model covering four core dimensions: institutional distance, market attractiveness, competitive intensity, and enterprise resource endowment. This model provides a systematic analytical framework for enterprises to scientifically evaluate target regions and select matching entry modes. Furthermore, the paper conducts an empirical application with Shanxi Anyu Construction Supervision Co., Ltd. as a case, demonstrating the feasibility of its strategy of selecting key western regions, adopting a "progressive" approach, and combining the risk response plan of "digital management and control + localized cooperation". This study not only provides specific strategic planning for Anyu Company but also offers decision-making references with both theoretical support and practical value for similar domestic regional supervision enterprises to break through geographical limitations and achieve high-quality development in the environment of the unified national market.