Abstract: To explore the cause of Potamogeton lucens’s decline, a dominant plant inhabiting the shallow water of Nansi Lake, the physiological and biochemical changes of P. lucens were examined under a group of constant temperatures at 25℃, 30℃, 35℃, and 40℃, respectively, for 3h. The results showed that the contents of protein, soluble sugar and chlorophyll decreased significantly, while the content of malondialdehyde (MDA) increased significantly at a high temperature above 35℃. The results indicated that high temperature above 35℃ had significant damage to P. lucens. The photosystem of P. lucens was more sensitive to heat stress. The characteristics of standardized chlorophyll fluorescence kinetics curves under heat stress were as follows. Peaks at J and K phases were observed, but no L-band was found on the normalized chlorophyll fluorescence kinetics curves. The chlorophyll fluorescence parameters were calculated from the OJIP curves of the heat-treated leaves. The results showed that the initial closing speed of the reaction center (dVG/dt_o, dV/dt_o) slowed down with the increase of temperature under heat stress, but it took a shorter time to reach the maximal fluorescence (T_fm). The maximum quantum yield of PSII (Photosystem II) photochemistry (F_v/F_m) decreased. However, the non-photochemical constants (K_n), relative variable fluorescence at the J-step (V_j), and dissipated energy flux (DI_o/RC, DI_o/CS_o, F_o/F_m) increased under heat stress. Although the turn-over number of Q_A (N), average redox state of Q_A (S_m/T_fm), and relative variable fluorescence at the I-step (V_i) barely changed, the plastoquinone pool (S_m) decreased significantly at high temperature. Absorption and trapped energy flux per RC (ABS/RC, TR_o/RC; reaction center, RC) increased, whereas the electron transport efficiency per RC (ET_o/RC) decreased when temperature increased. Heat stress also decreased the trapped energy flux, electron transport flux and density of RCs per CS (TR_o/CS_o, ET_o/CS_o, RC/CS_o; cross section, CS). These effects of heat stress on photosystem eventually led to a significant reduction in the structure and function index (SFI_abs), performance index (PI_abs), and drive force for photosynthesis (DF) of the P. lucens leaves. These results demonstrated that heat stress mainly caused inactivation of oxygen-evolving complex of PSII, reduction of the density of RCs, and decrease of photochemical efficiency of RC in P. lucens plants, and these led to the production of reactive oxygen species, and thus caused remarkable damage to cells. Therefore, P. lucens is a sensitive aquatic plant to high temperature in summer.