An understanding of regulating factors and early warning of Ulva prolifera biomass increase may reduce harm or prevent bloom disasters in the Yellow Sea. We investigated the minimum nutrient concentration and light-limiting depth (Z_lim) for the growth of floating U. prolifera thalli. Bioavailable dissolved nitrogen (BDN) concentrations in most parts of the study area were almost always higher than the minimum N concentrations required for the growth of floating thalli, indicating no N limitation for the growth of floating thalli. However, the minimum N concentration required for the development of germlings into thalli was higher than BDN in the majority of the area north of 35°N. This indicated that germlings floating out of Subei Shoal were unable to grow into thalli because of N limitation. The minimum P concentration required for germling development was higher than the total dissolved P north of 35°N. This suggested that P limitation occurred for germlings floating out of Subei Shoal. The Z_lim for the floating thalli was <0.1 m in most parts of Subei Shoal, which explained why the rapid growth of floating thalli only occurred when they floated out from the Subei Shoal. A grid pattern with the phased multiple increase in biomass per day was designed to predict the possible accumulated multiple increase in biomass (AcMp) when U. prolifera drifted northward following different trajectories. The predicted AcMp values in 2017, 2010, and 2009 were close to the ratio of the coverage area from remote sensing data. Such a grid pattern facilitates quick decisions in disaster prevention and reduction.