To improve the utilization of fertilizer and water resource at the same time, a new slow-release potassium silicate fertilizer with the function of superabsorbent and water retention (SRPS) was prepared, which possessed the core/shell structure. Its core was potassium silicate in an alginate matrix, and the shell was poly(acrylic acid-co-acrylamide)/kaolin (P(AA-co-AM)/kaolin) superabsorbent polymer. The product contained 26.6% K2O and 10.4% SiO2. The effects of the amount of acrylamide, crosslinking agent, initiator, degree of neutralization of acrylic acid (AA), and kaolin concentration on water absorbency were investigated and optimized. The water absorbency of the product was 85 times its own weight if it was allowed to swell in tap water at room temperature for 90 min. Release characteristics of potassium and silicon from SRPS in soil and water-retention capacity of the soil with SRPS were also investigated. A non-Fickian diffusion mechanism of nutrient release was proposed, and the release rate factor K, the release exponent n, and the diffusion coefficient D of nutrients in SRPS was calculated. This product with good slow-release and water-retention capacity, being nontoxic in soil and environmentally friendly, could be especially useful in agricultural and horticultural applications.