Designing frame structures for large array antennas is challenging, requiring high stiffness and strength under strict weight limits and complex boundary conditions, while addressing internal component layout, installation, routing, and manufacturability. Traditional empirical methods fall short, necessitating advanced optimization approaches. This study develops a topology optimization method based on explicit boundary description for large-scale array antenna frames. The method enables condition- and dimension-specific optimization, providing optimal load transfer paths and improving structural performance. Compared to empirical designs, the optimized side frame achieves a 16% mass reduction and 14% lower deformation; the middle frame shows a 12% mass reduction and 14% stress reduction; and the back-structure demonstrates a 27.4% mass reduction and 31.8% stress reduction. The proposed method significantly reduces design redundancy, enhances efficiency, and is applicable across diverse conditions. It offers a robust tool for lightweight, high-performance antenna frame design, reducing costs and advancing large-scale array antenna engineering.