The invention discloses a root-inhibiting water retention agent for a planted roof. The root-inhibiting water retention agent contains a mixed solution of triiodobenzoic acid, mepiquat chloride and chlormequat chloride and a water retention agent, wherein in the mixed solution, the concentration of the triiodobenzoic acid is 1.0*10<-7> to 1.5*10<-5> mol/L, the concentration of the mepiquat chloride is 1.0*10<-4> to 1.0*10<-2> mol/L, the concentration of the chlormequat chloride is 5.0*10<-5> to 5.0*10<-3> mol/L, and the mass/volume ratio of the water retention agent to the mixed solution is 3.57g: 100L. According to the root-inhibiting water retention agent for the planted roof, roots of plants can be effectively inhibited from growing towards a waterproof layer of a building, and the growth of the upper parts of the plants is mainly affected by nutrient soil of an upper layer, so that lateral roots of the plants grow extensively, and the influence on the upper parts of the plants caused by a root inhibiting agent is retarded; meanwhile, due to the water retention agent, the number of times of watering can be obviously reduced, and the interval of watering is prolonged, so that water resources are saved, and the cost for manual watering is reduced.