The invention provides a water retention agent suitable for arid/semiarid region farmland soil. The invention is technically characterized in that the water retention agent is prepared from 2-8 parts of polyvinyl alcohol, 0.5-1 part of dodecyl fatty alcohol polyoxyethylene (3) ether ammonium sulfate, 0.1-0.5 part of dodecyl alcohol polyoxyethylene (9) ether, 0.001-0.008 part of triacontanol, 0.1-0.5 part of n-octanol, 5-15 parts of polyacrylic acid, 2-10 parts of ammonium monoacid phosphate, 2-5 parts of potassium sulfate, 2-10 parts of fulvic acid, 0.01-0.05 part of indolebutyric acid, 1-3 parts of magnesium sulfate and 40-55 parts of water. The water retention agent is prepared by the following steps: weighing the components in parts by weight, mixing at the room temperature of 20-25 DEG C, sufficiently stirring for 40-60 minutes to form a uniform mixture, and filtering to obtain the finished product. After the amount of precipitation reaches more than 20mm, the water retention agent is sprayed on the ground surface with a sprayer. The invention is suitable for the demands of soil loosening and water retention for arid/semiarid region farmland soil, removes the soil hardening, loosens the soil, improves the air permeability, promotes the formation of the crumb structure, breaks the farmland plough pan, increases the plough layer depth, enlarges the storage capacity of the soil reservoir, and enhances the water-holding capacity of soil.