Abstract: Supplemental far-red light has been widely used to regulate plant growth and development in vertical farms. There is an overall positive effect of the far-red on plant growth and yield. However, photosynthesis can be reduced after far-red exposure. Currently, it is still lacking in the contribution rate of the far-red to the photosynthesis and growth of plants in indoor production systems. This study aimed to quantify and then evaluate the photosynthesis and growth responses of lettuce grown under different levels of supplemental far-red. Lettuce plants were also cultivated under a photoperiod of 16 h with a photosynthetic photon flux density of 250 μmol/(m²·s) of either white light (CK) or Red to Far red (R:FR) ratio of 1.6 (T1) or 0.8 (T2). A systematic measurement was performed on the plant morphological traits, leaf photosynthesis, as well as the fresh and dry weight after treatment. 3D plant models of the lettuce plants were then recreated using 3D scanning technology. The plant photosynthesis and biomass production were simulated by a 3D modelling and ray-tracing approach. The results showed that the fresh and dry weights of both shoot and root increased with the decrease of R:FR ratio. The shoot fresh weight of the T1 and T2 significantly increased by 10.2% and 23.3%, respectively, compared with the plants under white light. At the same time, the shoot dry weight of the T1 and T2 significantly increased by 15.9% and 39.5%, respectively (P<0.05). Besides, the plant height and leaf area in the T1 treatment increased by approximately 28.9% and 44.3%, respectively, compared with the CK. T2 treatment further improved the plant height and leaf area by 49.5% and 83.6% compared with CK. Leaf photosynthetic parameters also depended on the light treatments. Among them, the J_max was significantly reduced under the T1 and T2 light treatment by 11.6% and 21.7%, compared with the CK. While the V_cmax was significantly reduced by 33.7% and 47.6%, respectively (P<0.05). Moreover, the R_d value in the T1 and T2 decreased by 18.1% and 38.7%, respectively, compared with the R_d under the white light. The curvature θ was not significantly related to the light treatments. And the g_m decreased with the increased FR light intensity. Nevertheless, the 3D model demonstrated that the plants with the supplemental far-red showed increasing light interception and whole-plant photosynthesis. The light capture of the T1 and T2 was significantly higher than the CK, increased by 42.7% and 68.2%, respectively (P<0.05). The highest whole-plant photosynthetic rate was found in the T2 treatment, followed by T1. The CK exhibited the lowest plant photosynthetic rate. The higher canopy light interception under low R:FR ratio was mainly attibutable to the increased plant height and leaf area. The contribution degree of plant height to light interception was 92% (T1) and 131% (T2), respectively, and the leaf area had the greatest impact on the fraction of light interception, whose contribution degree was 113% (T1) and 145% (T2), respectively. The leaf photosynthetic traits were caused by the reduced R:FR ratio (1.6 and 0.8). There was a decrease in the fraction of the canopy's gross photosynthesis by 11.3% and 14.5%, respectively. The plant architectural trait resulted from the reductions in the T1 and T2, leading to an increase in canopy photosynthesis by 41.6% and 45.9%, respectively. In general, the two influencing factors were evaluated on the positive impact of the plant architectural trait versus the negative impact of the photosynthesis of the leaf. The overall photosynthesis of the entire plant was significantly enhanced after the evaluation. All trait responses to the FR intensity also increased photosynthesis by 20.6% and 23.5%, respectively. Therefore, the far-red light-induced morphologies enhanced the light interception and the whole-plant photosynthesis, thereby increasing the lettuce yield.