Abstract: Algal growth and photosynthesis are susceptible to Hg stress. To screen for Hg-tolerant Chlamydomonas reinhardtii strains, the growth, chlorophyll a content, and chlorophyll fluorescence activity of 11 Chlamydomonas reinhardtii strains were compared at different Hg concentrations. The results showed a significant decrease in chlorophyll a content d and inhibited growth as mercury ion concentration increased. Parameters such as V_j and M_o increased rapidly, while ψ_o and φE_o decreased rapidly. It indicated a disruption in the electron transfer between the donor and acceptor side of the photosynthesis system Ⅱ (PSⅡ) reaction center, leading to inhibition of photosynthetic activity. Among the 11 Chlamydomonas reinhardtii strains, Chlamydomonas reinhardtii FACHB-889 displayed the lowest tolerance to Hg (EC₅₀ of 0.762 mg/L), whereas Chlamydomonas reinhardtii FACHB-2217 demonstrated the highest tolerant (EC₅₀ of 2.848 mg/L). Under low mercury concentration stress (<0.5 mg/L), PSⅡ activity of Chlamydomonas reinhardtii FACHB-889 was significantly reduced (P<0.05) compared to the control, and there was no significant difference in the activity of photosynthetic system Ⅱ (PSⅡ) of Chlamydomonas reinhardtii FACHB-2217. Under high mercury concentration stress (>1.0 mg/L), Chlamydomonas reinhardtii FACHB-889 inhibited photosynthetic activity, with a notable decrease in the number of active reaction centers per unit area (P<0.05). This resulted in the inefficient utilization of absorbed energy by photosynthesis, leading to abnormal energy accumulation and cell inactivation or death. In contrast, Chlamydomonas reinhardtii FACHB-2217 showed a significant increase in energy absorbed (ABS/RC), energy used for Q_A reduction (TR_o/RC), and energy used for electron transfer (ET_o/RC) in the active unit reaction centers (URCs), and the cells were able to efficiently convert the absorbed energy through the increased photosynthetic capacity of active URCs. energy and eliminating the negative effects of partial reaction center inactivation. The study suggests that differences in photosynthetic activity and energy partitioning are important factors in the sensitivity/tolerance of Chlamydomonas reinhardtii to Hg.