Abstract In order to investigate the effects of the combination of marine calcium fertilizer and Bacillus subtilis on tobacco growth and photosynthetic characteristics, and to solve the problems of poor soil structure stability and low soil fertility in tobacco planting in Ruyuan County, as well as to explore the effects of the combination of marine calcium fertilizer and Bacillus subtilis on soil aggregate structure, microbiomass carbon, and soil enzyme activity, this experiment took the red fire soil in Guangdong tobacco planting area as the experimental object, and set up four treatments: conventional fertilization (CK), marine calcium fertilizer (1500 kg/hm ², T1), Bacillus subtilis (3000 g/hm ², T2), and the combination of marine calcium fertilizer and Bacillus subtilis (1500 kg/hm ²+3000 g/hm ², T3). The results showed that T3 treatment significantly improved the agronomic and photosynthetic characteristics of tobacco, significantly reduced soil pH, and increased the content of EC, total nitrogen, total phosphorus, available phosphorus, and available potassium in the soil. Compared with CK, T3 treatment increased root dry weight, root fresh weight, and root vitality by 46.28%, 42.05%, and 95.62%, respectively; The plant height, leaf length, and leaf width are optimal at all stages, with a 23.94% increase in stem circumference during the mature stage; The net photosynthetic rate, stomatal conductance, transpiration rate, and SPAD value were significantly higher than other treatments, and the SPAD value at maturity was 16.35% higher than that of CK. In terms of tobacco quality, under the B2F grade, the chlorine and potassium content of T3 treatment increased by 15.54% and 8.49% respectively compared to CK, while the sugar alkali ratio and nitrogen alkali ratio decreased; Under the C3F level, T3 has the best total sugar, reducing sugar, and quality indicators. The proportion of high-quality tobacco and theoretical output value in T3 treatment were significantly higher than those in other treatments. Compared with CK, T3 treatment increased soil exchangeable calcium and magnesium by 46.3% and 220.2%, respectively. The soil bulk density decreased by 14.91%, the soil moisture content increased by 57.26%, the proportion of soil super large aggregates and large aggregates increased, and the proportion of small aggregates decreased. The soil MDC, GWD, and R0.25 were significantly increased compared to CK. After combined application, the soil SOC content was significantly increased, and the MBC content was significantly increased compared to the application of marine calcium fertilizer or Bacillus subtilis alone. The soil catalase, urease, sucrase, and alkaline phosphatase increased by 10.19%, 21.21%, 36.41%, and 8.65%, respectively. Recommend using a combination of marine calcium fertilizer and Bacillus subtilis (1500 kg/hm ²+3000 g/hm ²). In summary, the combination of marine derived calcium fertilizer and Bacillus subtilis can effectively promote the growth of tobacco plants, improve their photosynthetic characteristics, and enhance the yield and quality of tobacco leaves. It also significantly improves the stability of soil structure and enhances soil fertility. Keywords: Marine-derived calcium fertilizer; Bacillus subtilis; Agronomic traits; Photosynthetic characteristics; Soil structure; Aggregate stability; Microbial carbon content; Soil enzyme activity.