Grassroots union alliances represent an innovative strategy by local unions to activate the agency of grassroots unions and have become an essential approach for union work in regions with concentrated industries. This study examines the phenomenon of spontaneously formed grassroots union alliances across three regions, investigating their formative dynamics and factors infl uencing sustainable operation. Comparative analysis reveals that the establishment of these alliances benefi ts from "top-down support" by regional higher-level unions, which provide organizational goals and resources. The alliances rely on "coordinators" to foster "strong ties" among union chairs and cultivate value alignment. Resource incentives from upper-level unions further enable organizational symbiosis. While such alliances enhance the agency of grassroots union chairs and the continuity of union work, the study concludes that without institutional safeguards, their effectiveness remains dependent on the presence of "coordinators".