In order to explore a novel type of agricultural water-retaining agent, borax crosslinked fenugreek galactomannan-borax hydrogel (FGB) was synthesized. The hydrogel was characterized by thermogravimetric analysis, FT-IR, XRD, and SEM. The largest swelling index of FGB (115) was higher than that of guar gum hydrogel (70) at pH 9. The water absorption capacity of sandy soil was studied by mixing the hydrogel with soil, then FGB was evaluated as a latent water retention agent. Results showed that the swelling index of sandy soil increased from 16.28% to 35.53% with 0.5 wt.% hydrogel added. The water retention time increased from 2 to 11.5 days at 20 °C and 60 % humidity. Furthermore, more than ten cycles of water absorption and water loss of soil-FGB were carried out at 50 °C at a relatively stable water retention capacity. This low-cost, environmentally friendly hydrogel has great application potential in agriculture for soil water conservation.