In the last three years, vast green tides occurred repeatedly in the Yellow Sea area every summer. The biomass of floating green tide algae increased fast in a few months, moreover, the area covered by floating green tide algae expanded sharply. The material of this study is the dominant species Ulva prolifera in green tide. The reproductive characteristics study of the floating green algae showed that every square centimeter of the thalli produced about 5.35×10⁶ spores or 1.07×10⁷ gametes; during May to August, 25% surface area of the mature thalli produced about 1.15×10⁷ spores or 2.31×10⁷gametes. Parthenogenesis and asexual reproduction are the main reproductive modes of the floating green algae. Gametes showed intensive positive phototaxis, after syngamy, motile zygotes showed negative phototaxis. The spores and gametes could grow to new thalli independently, and the same to motile zygotes. As a result, the strong reproductive capacity of floating green tide algae Ulva prolifera might be the main reason of green tide occurrence.