To investigate the impact of portal isolation piles on surface settlement during the construction of large-diameter shield tunnels, theoretical derivation, numerical simulation, and in-situ testing were utilized, based on Section 2 of the Shanghai Airport Link Line. Firstly, the ground settlement in free fields was analyzed by comparing numerical models with analytical formulas. Additionally, the effects of three types of isolation piles were compared, including single-sided (verified by existing analytical formulas), double-sided, and portal type. Finally, in-site test of ground settlement was conducted, and the following conclusions were drawn. Through comparison and theoretical verification, it was determined that portal isolation piles provide the most effective control over ground settlement, with successful application results achieved in engineering practice. As tunnel construction progresses within portal isolation piles, an increasing trend in ground settlement near the high-speed railway bridge was observed, while a slight uplift in ground settlement farther from the bridge was noted. The maximum ground settlement was found along the tunnel centerline. Within the isolation piles, settlement increases closer to the tunnel center, with a significant rise on the side adjacent to the isolation piles. Outside the isolation piles, settlement gradually decreased with increasing distance. However, a plateau section appeared within a range of half the width of the portal isolation piles.