Red and blue light are the main active spectra of plants for photosynthesis and photomorphogenesis. Also red and blue light-emitting diodes (LEDs) have been the dominant light source for plant factory. Therefore, the response characteristics and mechanism of plants to continuous light of red and blue spectrum need to be explored. In order to realize the application of continuous lighting in plant factories, effects of LED red and continuous blue light (CL) on lettuce growth and nutrient absorption before harvesting were studied using ICP-AES technology in an environmentally controllable plant. Under the light intensity of 150 μmol·m-2·s-1, three kinds of red and blue light were set up: 2R∶1B (Q2∶1), 3R∶1B (Q3∶1) and 4R∶1B (Q4∶1), and two nitrogen forms: 80% nitrate nitrogen (N80%) and 100% nitrate nitrogen (N100%). The results showed that: The interaction between LED light quality and nutrient liquid nitrogen form had a significant effect on the dry shoot weight of hydroponic lettuce and had no significant effect on the fresh weight of the ground and the dry weight of the root before CL. There was no significant effect on the content and accumulation of N, C, P, K, Ca, Mg, Fe, Mn, Cu and Zn before CL. After CL, the interaction between light quality and nutrient liquid nitrogen form had significant effects on fresh root weight and dry root weight of hydroponic lettuce, and there was no significant difference in the effect of the fresh shoot and dry weight, only the content of N and P, N. The accumulation of P, Fe and Zn had a significant effect. CL had a significant effect on the biomass, nutrient elements content and accumulation of hydroponic lettuce. Comparing with pre-CL, the fresh shoot weight, fresh root weight, dry shoot weight and dry root weight increased significantly. The content of each nutrient element decreased in different degrees, and CL significantly reduced the contents of N, P, Fe and Zn, the content has no significant effect on the contents of C, K, Ca, Mg, Mn and Cu. CL significantly increased the accumulation of N, C, P, K, Ca, Mg, Fe, Mn, Cu, and Zn. In summary, LED light quality and nutrient solution nitrogen form treatment had a significant effect on the dry weight of the ground and had no significant effect on the content and accumulation of various nutrients before CL. The light quality of LED and nitrogen form of the nutrient solution had significant effects on the fresh and dry weight of root and had a significant effect on the content of N and P, the accumulation of N, P, Fe and Zn after CL. Compared with pre-CL, the fresh shoot weight, fresh root weight, dry shoot weight and dry root weight of hydroponic lettuce increased significantly after CL treatment. CL significantly reduced the contents of N, P, Fe and Zn, but N and C contents were significantly increased. The accumulation of P, K, Ca, Mg, Fe, Mn, Cu, and Zn were increased. In conclusion, the combination of cultivation light quality & nitrogen form (i. e. N80%Q4∶1) with pre-harvest continuous lighting by LED red and blue light is an optimal strategy for improving contents and accumulation of some kinds of nutrient elements in lettuce.