Abstract: In this study, the diurnal change of photosynthesis activity in response to various tidal patterns, the relationship between photosynthetic activity and tissue water content, and the interactive effect of desiccation and irradiance on photosynthetic activity in Gloiopeltis furcata were investigated by using portable pulse amplitude modulated (PAM) fluorometer. Results showed that F_v/F_m decreased more rapidly during the noon low tide than during the morning or evening low tide. F_v/F_m decreased slowly at the beginning of desiccation during the morning low tide, but decreased rapidly throughout the evening low tide. F_v/F_m recovered to the initial values on the same day no matter when the low tide occurred, suggesting the occurrence of dynamic photoinhibition. These features endowed G. furcata with an ability to adapt to the periodic desiccation on high intertidal rocks. The maximum (F_v/F_m) and effective (Φ_PSⅡ) quantum yield declined with the decrease of tissue water content (TWC). However, photosynthetic activity could recover completely when TWC exceeded 6%, showing a strong ability of G. furcata to tolerate desiccation. The relationships between TWC and F_v/F_m and Φ_PSⅡ were as follows: F_v/F_m=0.68+(0.0044-0.68)/\[1+(TWC/66.96)⁵\], R²=0.99; Φ_PSⅡ=0.585+(0.004-0.585)/\[1+(TWC/ 73)¹⁰\], R²=0.99. ANOVA result further showed that the interactive effect of irradiance and desiccation on photosynthetic activity was significant, and that the photoinhibition degree increased with elevation of irradiation and duration of desiccation. The extreme condition (6 h desiccation at 1000 μmol photons·m^-2·s^-1) resulted in a serious photoinhibition, with the longest period of complete recovery for photosynthesis activity.