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The Yellow Sea green tide (YSGT) is the world’s largest transregional macroalgal blooms, and the causative species <i>Ulva prolifera</i> (<i>U. prolifera</i>) suffers from ultraviolet-b radiation (UVBR) during the floating migration process. Previous study confirmed that <i>U. prolifera</i> displayed a wide variety of physiological responses characterized as acclimation to UVBR, while the response mechanisms against low-dose and short-term radiation (LDSTR) are not clear. A study with photosynthetically active radiation (PAR) and UVBR was designed: normal light (NL: 72 μmol photons m⁻² s⁻¹), NL+0.3 (UVBR: 0.3 W·m⁻²), and NL+1.6 (UVBR: 1.6 W·m⁻²). The results showed that high-dose UVBR inhibited photosynthesis in thalli, especially under long-term exposure, while a variety of physiological responses were observed under LDSTR. The inhibition of photosynthesis appeared to be ameliorated by the algae under LDSTR. Further analysis showed that <i>U. prolifera</i> achieved balancing damage by means of non-photochemical quenching (NPQ), accumulation of phenolic compounds coupled with the ASA-GSH cycle involved in the antioxidant process and enhanced photorespiratory metabolism under LDSTR. This study provides new insights into the balancing damage mechanisms of <i>U. prolifera</i> under LDSTR, enabling the thalli to adapt to the light conditions during the long duration and distance involved in floating migration.