Rocky slopes lack the necessary conditions for plant growth, i.e., soil and water, making vegetation recovery difficult and necessitating artificial soil conditions for vegetation establishment. Water-retaining agents are essential functional substances for constructing artificial soil on rocky slopes. In this study, the effect of water-retaining agents on the growth of artificial vegetation and soil nutrient loss was investigated by setting different application rates of water-retaining agents (10 g/m², 20 g/m², 30 g/m², 40 g/m², and 50 g/m²). Artificially simulated rocky slope conditions were achieved using the guest soil spraying technology after hanging the net, which provided support for the application of the water-retaining agent. Results showed that the contents of extractable nitrogen (EN), extractable phosphorus (EP), and extractable potassium (EK) in the soil, the number of plant individuals per unit area, plant height, and ground diameter of the water-retaining agent treatment were significantly higher than those of the control group (set up with no water retention as the control group, hereafter “CK”). By contrast, the nutrient content in the runoff fluid was significantly lower than that of CK. The application of the water-retaining agent significantly enhanced the soil’s ability to retain water and fertilizer, thus improving the growing environment and plant growth. Under the conditions of this experiment, the application rate of a 40 g/m² water-retaining agent is reasonable from the perspectives of economic cost, soil nutrients, and plant growth.