Pyropia species were the most popular algae cultivated in China with great commercial importance. Yellow spot disease (YSD) was a high-incidence infectious disease in the conchocelis filament sporing stage of Pyropia, which seriously affects the production. In this paper, the free living conchocelis filament of Pyropia haitanensis were infected by Vibrio mediterranei 117-T6, a YSD pathogen. The effects of environmental factors on the growth of the strain and its infectious ability were studied. The activities of SOD, POD and the contents of phycobiliprotein, chlorophyll a (Chl. a), soluble protein, free proline (Pro) and malondialdehyde (MDA) in the algae were measured under susceptible infectious conditions. The results showed that the optimum growth conditions of V. mediterranei 117-T6 were 30 ℃, pH 7.0 and salinity 20, while the most susceptible infectious conditions were temperature 30 ℃, pH 6.0 and salinity 20. After it was infected by V. mediterranei 117-T6 under the susceptible infectious conditions for 12 hours, the activity of SOD, POD, the content of phycobiliprotein, soluble protein and free proline in P. haitanensis conchocelis filaments were significantly higher than those in the control group (P<0.05). The content of MDA reached the highest value at 6 hours and was significantly higher than that in the control (P<0.05). After being infected for 24 hours, the content of Chl a in the infectious group achieved the highest, which was significantly higher than that in the control group (P<0.05). The study showed that in a short period, V. mediterranei 117-T6 could stimulate the algae to produce stress-response reactions. However, with the estension of the infection, the reactive oxygen species and osmotic pressure in the algae caused by pathogen increased and finally could lead to the death of Pyropia. Adverse environmental factors such as high temperature and acidification can exacerbate the infectious ability of V. mediterranei 117-T6, and the secondary factors were temperature, pH and salinity. The results suggested that the comprehensive treatment of Pyropia YSD should be carried out from the two aspects: the environment and the pathogenic bacteria.