Abstract: To address the issues of uncontrollable rib waviness and higher-order strip shape defects when rolling ultra-high-strength steel with wide widths on an 18-high single-stand mill, this paper establishes finite element models for both the 18-high single-stand mill and CVC cold mill and systematically compares their strip shape control characteristics. The study finds that while the transverse stiffness of the roll gap in the 18-high single-stand mill is smaller than that of the CVC cold mill, its bending roll control capability and roll shifting regulation effectiveness are superior. Based on these findings, the paper proposes three core optimization measures:first, designing a segmented adaptive tapered intermediate roll with a 250 mm shifting stroke to achieve precise edge pressure control for multi-width strip; second, optimizing the large fillet transition zone of the tapered section and adopting a sine curve grinding process to effectively mitigate stress concentration and improve strip shape quality; third, innovatively proposing a "bending roll dominance-shifting roll compensation" primary-secondary collaborative control strategy, where 76% of the symmetric quadratic strip shape deviation is regulated by the faster-responsive bending rolls, while only 24% is compensated by roll shifting, fundamentally suppressing rib waviness defects caused by excessive roll shifting. This research provides an effective theoretical foundation and engineering solution for addressing the "bottleneck" plate shape challenges in producing ultra-high-strength steel with 18-high mill.