Abstract: Continuous rainfall has directly caused changes in light intensity and sea surface salinity. In recent years, the cultivated output of Gracilariopsis lemaneiformis in Fujian Province has fluctuated greatly effected by the rainy season. To reveal the mechanism, three salinity gradients (lower salinity, LS: 16; medium salinity, MS: 24; higher salinity, HS: 32) and two light intensity levels (lower light intensity, LL: 30 μmol/(m²·s); higher light intensity, HL: 120 μmol/ (m²·s)) were set to study the effects of 6 different treatment combinations on the growth and photosynthetic characteristics of G. lemaneiformis in this experiment. Results showed that firstly salinity, light intensity and their interaction had significant effects on the relative growth rates (RGR) and net photosynthetic rate (P_n) of G. Lemaneiformis (P<0.05). Appropriate reduction of salinity (MS) and increment of light (HL) were beneficial to the thalli RGR and P_n, while MS significantly inhibited respiratory rate (P_d). Secondly, light intensity and salinity had significant effects on effective quantum yield ［Y(II)］, maximum relative electron transfer rate (rETR_max), light energy utilization efficiency (α) and saturation irradiance (E_k) of G. Lemaneiformis (P<0.05). Under HL, Y(II) and rETR_max increased firstly and then decreased with increasing salinity. The interaction between light intensity and salinity had only insignificant effect on α (P>0.05). The last, salinity had a significant effect on the content of photosynthetic pigments of G. Lemaneiformis (P<0.05). Under the same illumination, the thalli pigments increased firstly and then decreased with the increase of salinity. Under MS, HL significantly promoted the synthesis of chlorophyll a, the interaction of salinity and light intensity had a significant effect on the chlorophyll a and phycoerythrin contents of G. Lemaneiformis (P<0.05). Further analysis showed that the appropriate reduction of salinity under sufficient light conditions promoted the algal photosynthesis and the synthesis of photosynthetic pigments, thereby promoting the growth of G. Lemaneiformis . When the salinity was too low (LS), the growth, photosynthesis and pigment synthesis of G. Lemaneiformis were significantly inhibited. Therefore, the cultivated output of G. Lemaneiformis in the rainy season may potentially depend on the changes of light intensity,sea surface salinity and their coupling effects.