In the context of China’s rapidly aging population, effectively expanding the coverage of the pension insurance system is essential to accelerating the development of a multi-tiered and multi-pillar old-age insurance system, improving social welfare, and fostering high-quality economic growth. The pension default mechanism, which replaces an “opt-in” with an “opt-out” arrangement, provides a novel approach to increasing pension coverage through interconnected design elements, such as automatic enrollment, auto-escalation, and qualified default investment options. Drawing on the “nudging” theory from behavioral economics, this study systematically reviews the theoretical foundations and strategic design of the pension default mechanisms. It focuses on the US and the UK—the first two countries to introduce such mechanisms—to analyze their specific implementation practices. This study finds that both countries have effectively overcome individuals’ limited rationality and inertia by adopting opt-out defaults and streamlining processes. Furthermore, two countries have successfully expanded their pension coverage through a combination of measures, including implementing automatic enrollment, setting minimum contribution rates, introducing automatic escalation of contribution rates, and offering default investment options. However, the application of pension default mechanisms has certain limitations. First, automatic enrollment relies on a clear employer–employee relationship and the existence of an employer-sponsored supplementary pension plan. Therefore, it struggles to effectively cover self-employed workers, those in new forms of employment, or employees of organizations without occupational pension plans. Second, the automatic enrollment mechanism may undermine the adequacy of future pension benefits due to passive inertia. Third, default investment mechanisms should ensure alignment with participants’ risk preferences and retirement goals. Accordingly, in light of China’s multi-pillar pension system, this study proposes a series of policy implications, including gradually and hierarchically implementing automatic enrollment alongside adjustments in contribution rates, moderately liberalizing individual investment choice, establishing qualified default investment alternatives, and promoting coordination between the second and third pillars through an integrated pension management platform, aiming to provide practical guidance for achieving high-quality development of the pension system.