At present, technologies of local application of phosphate fertilizer and biological stimulants have both successfully improved vegetable yields. However, whether there is a synergistic effect between them and the related mechanism of synergistic enhancement have not been reported. This study aims to take into account the characteristic of weak-developing root in vegetables, and based on the plastic response of roots to local phosphorus application, then further strengthen root development by adding biostimulant, carboxymethyl chitosan to the rhizosphere. Thus, the phosphate fertilizer and soil residual phosphorus can be fully utilized to achieve vegetable yield improvement. The amaranth was used as the vegetable varieties tested in the pot experiment with six treatments: mixed application of phosphate fertilizer (CK1), local application of phosphate fertilizer (CK2), and local application of phosphate fertilizer combined with carboxymethyl chitosan in four different concentrations (T1, T2, T3, and T4). The results showed that the exhibition in treatment of local phosphate fertilizer application combined with 100 mg/kg carboxymethyl chitosan (T2) were always the highest in yields, shoot phosphorus accumulation, shoot and root growth, available phosphorus in the rhizosphere soil and soil phosphorus phosphatase activity among all treatments. Compared with the mixed application treatment, the treatments of single local application of phosphate fertilizer and the local application of phosphate fertilizer combined with carboxymethyl chitosan could significantly increase the yield and shoot phosphorus accumulation of amaranth. The yield of T2 treatment significantly increased 61.5%、27.6% than that of CK1 and CK2 treatments, respectively, as well as their shoot phosphorus accumulation significantly increased by 71.0% and 31.5%, respectively. The root development indicators of T2 treatment were significantly higher than those of the mixed application and single local application of phosphate fertilizer treatments in the middle growth period and at harvest. At harvest, the root dry weight, total root length, total root surface area, total root volume, root average diameter, and root tips of T2 treatment were significantly higher than CK1 treatment by 79.3%, 23.5%, 27.3%, 29.5%, 10.5%, and 39.8%, respectively, as well as were significantly higher than CK2 treatment by 28.8%, 16.6%, 13.6%, 23.8%, 8.82%, and 26.6%, respectively. In addition, the shoot growth indicators of plant height, stem diameter, and shoot dry weight in T2 treatment in the middle growth period and at harvest were significantly promoted compared with CK1 treatment. The activities of acid phosphatase and alkaline phosphatase in the rhizosphere soil of T2 treatment were significantly higher than those of CK1 and CK2 treatments by 98.3%, 43.2% and 45.1%, 25.6%, respectively. The T2 treatment effectively accelerated soil phosphorus turnover, with its available phosphorus contents in the rhizosphere soil of T2 treatment significantly increased by 28.0% and 11.2% compared to CK1 and CK2 treatments, respectively, as well as its phosphorus activation coefficients significantly improved by 31.7% and 13.2% than CK1 and CK2 treatments, respectively. This research offers novel insights and technical support for advancing green sustainable phosphorus nutrient management in vegetable fields, primarily through enhancing vegetable root plasticity associated morphological and physiological changes.