Abstract: In laboratory simulated environment, we studied the seasonal impacts of the decomposition of Gracilaria confervoides in Daya Bay on the nutrient concentrations and dissolved oxygen in overlying water. According the climate of Guangdong coast, three control groups were designed: spring and autumn in 20 ℃, summer in 28 ℃, winter in 15 ℃, and the average sea water flow rate was 0.24 m·s^-1. The results show that dissolved oxygen (DO) decreased gradually with increasing temperature, and DO was negatively correlated to the biomass of G.confervoides in the water. In the decomposition process of Gracilaria, TN and TP values in overlying water ascended and then declined; the concentrations of NH₄⁺-N, NO₂^--N and NO₃^--N reached the maximum in synchronization. Furthermore, the phosphorus released prior to nitrogen. When in winter (15 ℃) and summer (28 ℃), the concentrations of nitrogen and phosphorus increased with increasing weight of Gracilaria, which was opposite in summer (28 ℃). The peak concentration of PO₄^3--P was also affected by seaweed weight, showing positive correlation, and the accumulation of both nitrogen and phosphorus in overlying water in winter (15 ℃) was obviously higher than that in summer (28 ℃). The results provide references for determining the density of seaweed in proliferation.