To construct urban three-dimensional transportation systems and reduce space occupation under bridges, the large cantilever-Y-shaped pier system was applied as a substructure of urban viaducts, with its application demand increasing year by year. However, related research on its reinforcement ratio under vehicle eccentric load has been rarely conducted. Based on an engineering case of an expressway construction project in Jiaxing City, Zhejiang Province, a 1∶5 scaled finite element model was established using Abaqus. The reinforcement ratios of Y-shaped piers in four models were set to 0.42%, 1.17%, 1.69%, and 2.29%, respectively. Among them, the reinforcement ratio of 0.42% was lower than the minimum reinforcement ratio (0.5%) stipulated in the Specifications for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts (JTG 3362—2018). To study the failure mode of the Y-shaped pier, numerical simulations were conducted on the above four models under vehicle eccentric load. The results indicate that: ① Under vehicle eccentric load, when the reinforcement ratio increases from 0.42% to 2.29%, the crack resistance of the pier column is not significantly improved, and the concrete cracking load at the Y-shaped turning area of each model is 64.8 kN; ② with the increase in reinforcement ratio, the concrete crushing location moves from the turning area at the waist on the compression side to the bifurcation on the compression side; ③ when the reinforcement ratio increases from 1.69% to 2.29%, the bearing capacity of the pier column is significantly improved, with an increase of approximately 29.85%.