The marine red algae Gracilaria/Gracilariopsis lemaneiformis were used as materials to study the effects of different concentration of salicylic acid on their growth and stress physiology under high temperature condition. The antioxidant enzyme activity, the osmotic regulation substances, phycobiliprotein, the features of chlorophyll fluorescence and HSP70 gene were studied when the algae grew in high temperature environment. Results showed that in the salicylic acid treatment group, the growth rate, enzymatic activity or membrane lipids damage degree were better than those of the control group at various degrees, and 5.0 and 10.0 μg/mL of salicylic acid treatment groups were obviously better than the other groups. The treatment with 10.0 μg/mL of salicylic acid showed the best effect with the daily relative growth rate increased 340% compared with the control group. When the treatment was on the 3rd day, each index of the 10.0 μg/mL group reached the maximum value, SOD increased 74%, POD increased 70%, CAT increased 40%, mannitol and proline increased 70% and 26% respectively. The content of phycoerythrin increased 46.2%, phycocyanin content increased 40%. Compared with the control group, the decrease of MDA content on the first day was the most obvious and decreased nearly 10%. The chlorophyll parameters of Fv/Fm, Fv/Fo, qP and ΦPSⅡ of G. lemaneiformis decreased under high temperature stress, and the variation tendency of NPQ increased first and then declined. It was found that 10.0 μg/mL of salicylic acid could increase the five kinds of chlorophyll fluorescence values compared with the control group. The expression level of HSP70 gene under high temperature was higher than those under normal conditions, while the expression level of high temperature group with salicylic acid treatment was lower than the control group. In conclusion, salicylic acid showed the effect of improving the high temperature resistance of G. lemaneiformis.