Abstract: Substrate cultivation has been characterized by the high yield and quality of tomatoes in recent years. Furthermore, biochar can be recognized as the soil conditioner to serve as the ideal additive in the substrate, due to the abundant porosity and the large specific surface area. This study aims to clarify the effects of irrigation and fertilizer coupling on the yield and quality of tomatoes cultivated in a biochar substrate. The biochar was incorporated into the substrate mixture that included agricultural wastes, such as distiller's grains and straw. A greenhouse experiment was conducted to investigate the effects of varying irrigation and fertilizer regimes on the leaf area index (LAI), soil and plant analyzer development (SPAD), plant nutrient uptake, N/K ratio, C/N ratio, yield, and quality of tomato cultivated in biochar substrate. The experiment consisted of two irrigation levels: I₁ (100% ET_c, where ET_c referred to crop evapotranspiration) and I₂ (80% ET_c); two fertilization rates (N-P₂O₅-K₂O): F₁ (240-180-200 kg/hm²) and F₂ (180-135-150 kg/hm²); and four amounts of biochar application: B₀ (0), B₁ (1%), B₃ (3%), and B₅ (5%). Furthermore, a structural equation model (SEM) was utilized to explore the driving factors of water-fertilizer coupling on the yield and quality of tomatoes cultivated in biochar substrate. Additionally, an entropy weight-TOPSIS model of multi-objective evaluation was also constructed with the targeting yield, quality, and their key driving factors. The optimal scheme was identified for the coupling of water and fertilizer with the biochar substrate. The results indicated that the biochar substrate had significant effects on LAI, SPAD value, nutrient uptake, yield, and quality of tomatoes (P < 0.05). Under the same conditions of irrigation and fertilization, the biochar substrate improved the potassium absorption and carbon assimilation of the whole plant, yield, and vitamin C content in the tomato plants while simultaneously reducing nitrate content. Furthermore, under the same fertilization rate, the yield decreased with the decrease of irrigation amount, and the soluble solids content, vitamin C content, and soluble sugar content increased with the decrease of irrigation amount. Compared with the I₁ treatment, the I₂ treatment resulted in an average decrease in yield by 13%, and an average increase in soluble solids content, vitamin C content, and soluble sugar content by 17%, 8%, and 11%, respectively. The SEM revealed that the carbon assimilation of the whole plant was the primary influencing factor on the yield, indicating a significant positive correlation (λ=0.96, P<0.001). The N/K ratio emerged as the principal regulatory factor for both nitrate content and the sugar-acid ratio, indicating a significant negative correlation with the sugar-acid ratio (λ=-0.66, P<0.01) and a significant positive correlation with nitrate content (λ=0.53, P<0.001). The SPAD value shared an indirect promoting effect on the yield and quality. According to the entropy weight-TOPSIS model of a multi-objective comprehensive evaluation, the I₁F₂B₃ treatment had the highest comprehensive ranking with a relative closeness degree of 0.76. Therefore, I₁F₂B₃ treatment can be the optimal coupling strategy of water and fertilizer and biochar substrate to synergistically improve yield and quality. This research can provide a theoretical basis to improve the utilization rate of agricultural waste and the effective management of irrigation, fertilizer, and biochar in the cultivation of high-quality vegetables.