In salt marshes, abiotic factors, interspecific competition or facilitation, and herbivory have been discussed extensively as important drivers of marsh plant distribution patterns. However, the relative importance of various regeneration processes across stress gradients received little attention, despite their importance in explaining plant community dynamics. In this study, we built a stage-structured plant assembly model to study how the influence of each regeneration process on plant assembly dynamics varies with environmental stress level, plant succession stage and initial seed availability, in a salt marsh in Yellow River Delta National Nature Reserve in China. We found plant assembly at higher saline conditions is mainly governed by stress tolerance regardless of the stage of succession and initial seed availability, as salinity stress can suppress biomass accumulation and propagule production of salt intolerant species. Under less saline conditions with high seed availability, juvenile competition plays the most important role throughout succession. When initial seed availability is low, dispersal is more important at the beginning, but juvenile competition is more important at later stage and determines the plant competition outcomes. Our findings imply seed availability is important in determining plant succession trajectories under less stressful conditions, and should be given more attention during restoration management in coastal marshes.