Abstract: To investigate the physiological responses and adaptation mechanisms of Enteromorpha to seawater salinity stress, a laboratory experiment with Enteromorpha linza and E. prolifera was conducted to study their fresh mass (FM), relative growth rate (RGR), relative electrical conductivity (REC), chlorophyll (Chl) and carotenoid (Car) contents, Chl a/Chl b, Chl/Car, chlorophyll fluorescence parameters, and osmotic adjustment ability (OAA) under the stress of different salinity levels of diluted and concentrated seawater for 10 days. Compared with the control, 10%-200% salinity seawater increased the FM and RGR of the two Enteromorpha species obviously, 100% and 50% salinity seawater made the FM and RGR of E. linza and E. prolifera peaked, respectively, while 300% salinity seawater decreased the FM and RGR of E. linza and E. prolifera significantly, with the decrement being larger for E. linza. The biomass of E. linza and E. prolifer only had an increase in 50% and 100% sanity seawater and in 10%, 50%, 100%, and 200% salinity seawater, respectively. The Chl and Car contents and Chl a/Chl b of E. linza and E. prolifera had a significant increase in 10% salinity seawater, but decreased after an initial increase with the increasing salinity level of seawater. The Chl and Car contents and Chl a/Chl b of E. linza and E. prolifera peaked in 100% and 50% salinity seawater, respectively. With increasing salinity of seawater, the light use efficiency (α), maximal photochemical efficiency of PSⅡ(F_v/F_m), actual photochemical efficiency of PSⅡ in the light (Yield), maximal relative electron transport rate (rETR_max), and halfsaturation light intensity (I_k) of E. linza and E. prolifera all showed the same variation trend as Chl. 10%-300% salinity seawater enabled E. linza and E. prolifera to express certain osmotic adjustment ability (OAA), and the OAA of E. linza and E. prolifer peaked in 100% and 50% salinity seawater, respectively.  The growth of Enteromorpha had no correlation with Chl/Car, but was significantly negatively correlated with REC and positively correlated with Chl, Car, Chl a/Chl b, F_v/F_m, Yield, rETR_max, α, I_k, and OAA. To sum up, 100% salinity was the optimal salt concentration for the growth of E. linza, and 50% salinity was optimal for E. prolifera. E. prolifera could adapt to a wider range of salinity than E. linza. The parameters REC, Chl, Car, Chl a/Chl b, F_v/F_m, Yield, rETR_max, α, I_k, and OAA could be used to evaluate the salt adaptation of Enteromorpha.