Ulva fasciata, an extremely rich wild seaweed in natural resources, has a high agricultural and medicinal value. They can also absorb large amounts of nitrogen, phosphorus and other nutrients, and can be used for aquaculture wastewater treatment and the culture environment improvement. With the rapid economic development, industrial pollution gradually increased, the effects of acid rain on terrestrial ecosystems and their harm have received widespread concern, while the effects of acid rain on the photosynthetic physiology of macroalgae in the intertidal zone have barely been studied. This study was to examine the effects of simulated acid rain on the physiological characteristics, including growth rate, chlorophyll content, chlorophyll fluorescence parameters, soluble protein and carbohydrate contents, and antioxidant activities, in hyposaline-grown alga Ulva fasciata. Three salinities (5, 10 and 25) and two levels of pH (4.4, 8.1) were set in the experiment. The water-pulse amplitude modulated fluorometer, liquid oxygenelectrode, and NBT, coomassie brilliant blue G250 and anthrone colorimetric method were used to determine the combined effects of simulated acid rain and salinity on the macroalgae U. fasciata. (1) The growth of U. fasciata was significantly inhibited by simulated acid rain (pH 4.4), and also affected by salinity of seawater. U. fasciata grown under moderate hyposaline (salinity 10) and normal pH(pH 8.1) showed the highest growth rate. The suppressive effect of simulated acid rain was enhanced by higher salinity. (2) Simulated acid rain (pH 4.4) significantly inhibited the maximum photosynthetic efficiency (F_v/F_m) and the effective photochemical efficiency (F_v'/F_m') of U. fasciata, but the salinity showed no significant effects. The inhibitory effects of simulated acid rain and hyposalinity on the maximum relative electron transfer rate (rETR_(max)) of U. fasciata have had also been observed. (3) Simulated acid rain (pH 4.4) reduced the dark respiration and photosynthesis oxygen release rate of U. fasciata, but showed no significant difference between salinity treatments. (4) Under normal salinity (25) and pH (pH 8.1) conditions, U. fasciata showed the highest SOD activity. Under relatively high salinity (10, 25) treatment, simulated acid rain inhibited the SOD activity of U. fasciata, while under salinity 5 treatment, simulated acid rain increased the SOD activity. (5) Compared with the treatment of pH 8.1, simulated acid rain significantly reduced the soluble protein content of U. fasciata, but the soluble polysaccharide content increased significantly, especially under hyposaline conditions. The effects of simulated acid rain on the physiology of U. fasciata were regulated by seawater salinity. Under the global climate change scenario, the increase of acid rain and salinity might influence the ecosystem of macroalgae grown in the intertidal niches, nearshore ponds and the estuaries.