Abstract: By using chlorophyll fluorescence technique, this paper investigated the photosynthetic parameters of seagrass Thalassia hemprichii under the coupled stress of Cu and Cd. Under the stress of Cd plus higher concentration (1.0 mg·L^-1) Cu, the maximum quantum yields (F_v/F_m), photochemical quenching (q_P), and nonphotochemical quenching (NPQ) declined remarkably, and the seagrass died after 5 days exposure, indicating that the photosynthesis of T. hemprichii suffered from severe damage, and the photoprotective mechanisms lost. On the contrary, after 5 days coupled stress of Cd and lower concentration (0.1 mg·L^-1) Cu, the q_P and NPQ had somewhat increase, and the F_v/F_m recovered to the preexposure level during recovery period, suggesting that the photoprotective mechanisms of T. hemprichii could be provoked and enhanced under the coupled stress of Cd and lower concentration Cu. In addition, under the coupled stress of Cu and Cd, the pigment content of the seagrass increased with increasing concentration Cd, suggesting that simply using pigment content to assess the stressed level of seagrass was inaccurate, and other indices and further analysis were needed.