Aiming at the problem of high energy consumption of artificial light source in plant factory, the study on improving the energy utilization rate of lettuce and reducing the energy consumption of light source was carried out through alternating light supply mode. Lettuce was grown in the fully artificial light plant factory, where adjustable red and blue LED panels were used as the sole light source for lettuce growth. In the experiment of alternating irradiation mode of red and blue light, the alternating interval in alternating treatments was 5min, 10min, 15min, 30min and 60min, respectively. The simultaneous RB light was regarded as the control, and also pure red R and pure blue B were set for comparison. The results showed that, the fresh weight, LUE and EUE of lettuce in all alternating light treatments were improved by respectively 18.6%~53.6%, 34.3%~78.6% and 34.6%~79.4% compared with the control. The highest fresh weight, LUE and EUE were all detected under 30min treatment, which were respectively 115.50g, 5.84% and 1.92%. R/B(30m) significantly improved the net photosynthetic rate, water use efficiency, Ψo, RC/CSo, PⅠabs, ABS/CS, TRo/CS and ETo/CS, as well as the specific activity of PSⅡ in lettuce leaves. Lettuce biomass were the highest under monochromatic red light, while LUE and EUE were significantly lower than that under R/B(30m). PⅠabs was the highest under pure blue light, while LUE and EUE were the lowest or not significantly different with the lowest value. The content of carotenoids in lettuce under R/B(5m) was significantly higher than that in other treatments. High alternating frequency of red and blue light better stimulated the synthesis and accumulation of carotenoids in leaves, which was beneficial for the protection of photosynthetic organs.