An unprecedented bi-macroalgal bloom caused by Ulva prolifera and Sargassum horneri occurred from spring to summer of 2017 in the western Yellow Sea (YS) of China, where annual large-scale green tides have prevailed for a decade. The distinct genesis and blooming dynamics of the two seaweed species were detected and described. Unlike the consistent raft-origin of the floating Ulva biomass, the massive pelagic S. horneri was derived from multiple sources (residual seaweeds from the previous winter bloom and those drifting from offshore water in the south). The scale of the green tide in 2017 was found smaller than the previous four years. We then discussed a number of hypotheses attributing to this reduction, including reduced epiphytic green algae from aquaculture rafts and the influences of the massive pelagic S. horneri. However, further research is needed to identify the origin of the pelagic S. horneri in the western YS and any affiliations with the benthic populations, and to elucidate the interactions of this species with the annual green tides and the ensuing consequences.