Abstract: Precise fertilization is often required for greenhouse vegetables in modern agriculture. However, conventional fertilization can rely mainly on the subjective experience. In this study, a systematic investigation was implemented to determine the relationship between leaf dry matter content and leaf phosphorus concentration in tomatoes. The field experiments were carried out under different phosphorus supply levels. A dilution curve model was then established, according to the leaf dry matter and phosphorus concentration data. A diagnostic analysis of the phosphorus nutritional status was constructed to recommend the fertilization amount for tomatoes. Both theoretical and practical guidance were provided for the phosphorus nutrition and supply in the substrate-cultivated tomato. The Jinpeng ‘101’ tomato variety was used as the plant material. Five levels of the phosphorus supply (0, 40%, 80%, 100%, and 140%) were designed according to the standard Yamasaki tomato nutrient solution formula. A systematic analysis was made to explore the effect of the phosphorus supply on the leaf dry matter content, leaf phosphorus concentration, phosphorus absorption, and single fruit weight, yield, and fruit quality. Dilution curve models were then established to determine the critical phosphorus concentration, critical phosphorus absorption, and phosphorus nutrition index (PNI) for greenhouse tomato. The analytic hierarchy process and entropy weight coefficient were used to determine the subjective and objective weights. Then, the tomato growth indicators were evaluated to combine with the technique for the order preference by similarity to ideal solution (Topsis) and entropy theory. Various growth indicators were also obtained to integrate with these weights using game theory. The evaluation scores were non-linearly fitted with the PNI in order to validate the accuracy of the PNI model. The results showed that the leaf dry matter content of the tomatoes significantly increased with the higher phosphorus supply levels, indicating that the appropriate phosphorus supply promoted the tomato growth. Furthermore, the leaf phosphorus concentration decreased gradually as the growth stage progressed. But the leaf phosphorus concentration shared an increasing trend at the higher phosphorus supply levels, indicating the essential role of phosphorus in tomato growth. Phosphorus absorption by tomato leaves also increased with the higher phosphorus supply levels, indicating a close relationship between phosphorus absorption and fertilization. In terms of the model construction, the critical phosphorus concentration dilution curve model demonstrated high accuracy, with a root mean square error (RMSE) of less than 0.0503 and a normalized RMSE (n-RMSE) of less than 10.499%. The strong stability and high predictive precision greatly contributed to the reliable theoretical basis for the phosphorus nutrition management in tomato. Phosphorus nutritional diagnosis with the PNI model showed that the 0 and 40% phosphorus supply treatments exhibited significant phosphorus deficiency, with the PNI values being below 1. The tomato growth was also limited by phosphorus. The PIN values were close to 1 in the 80% and 100% phosphorus supply treatments, indicating sufficient phosphorus nutrition with no significant growth limitations. The 140% phosphorus supply treatment (excess phosphorus) shared the PNI values greater than 1, indicating that the excessive phosphorus supply was negatively correlated to the tomato growth. Nonlinear fitting of PNI was evaluated among the treatments. The best performance was achieved in the 100% phosphorus supply. In the Guanzhong area of Shaanxi Province, the phosphorus supply was recommended for the substrate-cultivated tomatoes in spring and autumn greenhouse as follows: The phosphorus supply levels were 9.369, 25.292, and 18.743 kg/hm² (spring) and 9.680, 12.240, and 5.997 kg/hm² (autumn), respectively, at the seedling, flowering and fruit-setting, and maturity stages. These phosphorus supply levels were aligned with the various growth indicators of tomatoes. The finding can also provide scientific support to optimize the phosphorus management in tomato production.