Abstract: 【Objective】1000 times diluent of seaweed extract (SWE) was added into different concentrations of Hoagland solutions to investigate the effects on the yield and quality of hydroponic lettuce and provide references for the highly effective cultivation of lettuce. 【Methods】Water culture test was conducted and 4 treatments were designed: 1/2 Hoagland solution (CK); 1/2 Hoagland solution + SWE (T1); 3/8 Hoagland solution + SWE (T2); 1/4 Hoagland solution + SWE (T3). 【Results】 Compared with CK, T1 and T2 showed no significant difference in fresh weight, but significantly increased dry weight and photosynthetic pigment content. The polyphenol, flavonoids and soluble sugar content in lettuce increased while nitrate content decreased in 3 SWE treatments, especially T2 and T3 could better improve the quality of lettuce. Compared with CK, under T2 and T3, soluble sugar content significantly increased by 40% and 32.06%, respectively, while nitrate content decreased 42.9% and 42.59%, respectively, and the polyphenol content increased by 65.99% and 65.06%, while DPPH free radical scavenging rate increased by 72.49% and 75.77%, respectively. Compared with CK, the fresh weight of lettuce was not affected by adding seaweed fertilizer into 3/8 Hoagland nutrient solution, while the fresh weight of lettuce decreased 22.34% by adding seaweed fertilizer into 1/4 Hoagland nutrient solution.【Conclusion】T2 and T3 can better improve the yield and quality of lettuce. Taking full account of biomass and application costs, 3/8 concentration of Hoagland solution with 1000 times diluent of seaweed extract (T2) might be optimal for the high-efficiency and high-quality cultivation of hydroponic lettuce.