Abstract: Wheat (Triticum aestivum L.) is listed as the 3rd major food crop worldwide. Wheats growing in indoor farming can serve as a promising option for speed breeding in future production. Light environment is one of the most important environmental factors for plant growth. Particularly, far-red (FR) light has attracted considerable attention in recent years. However, the optimal light parameters, application timing, and underlying physiological mechanisms remain in speed breeding in plant factories. This study aims to optimize spring wheat breeding in plant factory using stage-specific far-red light application. The key developmental stages were identified for FR intervention. The stage-specific responses of spring wheat to FR light also provided a theoretical basis for the precise environment in indoor wheat cultivation. A FR application was then developed for the rapid growth of spring wheat indoor. A systematic investigation was implemented to explore the effects of FR application at different developmental stages on spring wheat growth, plant phenotype, yield, grain filling and seed quality. A field test was conducted in a plant factory in Shunyi district, Beijing from September 2024 to March 2025. Spring wheats (Jinqiang 12) were grown in pots (225 plants/m²) with substrate cultivation and drip irrigation under full-spectrum white LED panels at photosynthetically photon flux density (PPFD) of 500 μmol/(m²·s) and photoperiod of 22 h. With no FR supplementation as the CK, three FR (40 μmol/(m²·s)) treatments were applied as FR applying at different developmental stages: from emergence to trefoil stage (BT+FR); from grain filling to mature stage (AF+FR) and whole growth stage (WS+FR) with 40 μmol/(m²·s). The results showed that the FR application significantly dominated the wheat morphology. The WS+FR treatment increased plant culm height by 9%, whereas the flag leaf area was reduced by 17%, compared with the CK. The FR light supplementation over the entire growth period significantly shortened the growth cycle of spring wheat. The time from sowing to harvest was reduced by 55 days, which was about 13 days earlier than the CK. This treatment, however, also led to a yield reduction of 42%, due primarily to the decrease in spike number and grains per spike. In terms of flowering, the FR supplementation before the tillering stage showed a similar effect over the entire growth period, where the flowering time was advanced to around 33 days after sowing. Furthermore, the FR application after the flowering stage also optimized the grain-filling process. The active grain-filling period was reduced by about 14%, whereas the average grain-filling rate increased by 23%, with a 5% rise in the thousand-grain weight, compared with the CK. Although full-season FR treatment increased the net photosynthetic rate to a peak of 22.7 μmol/(m²·s) at the heading stage, there was a decrease in chlorophyll content and accelerated leaf senescence, resulting in reduced photosynthetic sustainability at the grain-filling stage. Additionally, the FR light significantly enhanced the grain protein content, with a 12% increase under full-season treatment, compared with the CK. While no significant effect was observed on starch content. In summary, the FR light also exhibited stage-specific regulatory on spring wheat, which accelerated the growth cycle to profoundly influence photosynthetic characteristics, dry matter allocation, and yield components. Precise timing of FR application mitigated the trade-off between early maturation and yield reduction. Specifically, the stage-specific regulation was summarized as follows: 1) The period from the emergence to the three-leaf stage (before the tillering stage) was the most sensitive for early flowering induced by FR. Supplementing 40 μmol/(m²·s) FR advanced flowering by 5 days. 2) Supplementing FR during the grain filling and the mature period (after flowering) was used to transport dry matter into the grains for the grain filling. 3) Supplementing FR during the whole growth stage was used to accelerate the whole growth cycle of spring wheat more than only application at certain stages. The optimal light environment (PPFD of 500 μmol/(m²·s) plus 40 μmol/(m²·s) FR and photoperiod of 22 h) was achieved to enter the full ripe stage of spring wheats within 55 days.