Abstract: In order to provide a theoretical basis for rational application of phosphate fertilizer, a sand culture experiment was conducted to study the change of several important physiological indexes at different growth stages of watermelon under different amounts of exogenous phosphorus. The results indicate that the chlorophyll value-SPAD values are decreased under the excess phosphorus at the vegetative growth period, and thus the crop photosynthesis is affected. The phosphorus deficiency symptoms are appeared at the seedling stage. The nucleic acid content of leaves is decreased at the flowering stage under the phosphorus deficiency, which means the growth and development of watermelon is inhibited if there is no sufficient exogenous phosphorus. The cellulose contents of stems are decreased under the excess supply of phosphate fertilizer, which results in plant resistance reduction. Different trends of the pectin contents at different growth stages are observed under different amounts of exogenous phosphorus.