The present invention discloses a method for preparing Erdite rod-shaped particles for water treatment by utilizing iron-containing waste mud in an underground water plant. In the method, aqueous iron mud is subjected to mechanical dehydration to obtain a first aqueous iron mud; sodium sulfide is added to the first aqueous iron mud to obtain a first mixture, wherein the ratio of the weight of the first aqueous iron mud to the volume of sodium sulfide is between 3% and 15%; water of an equal volume is added to the first mixture, and the obtained solution is heated in an airtight manner to 140° C.-270° C.; centrifugal treatment is performed to obtain a supernatant and a solid, and the solid is subjected to vacuum drying at 40° C.-60° C. for 24 h to obtain Erdite rod-shaped particles. In the method, aqueous iron mud formed after precipitation of backwash wastewater is directly used, no drying treatment is needed, and silicon and aluminum and other impurities do not need to be removed, thereby saving costs; Na2S is directly added to iron mud which is subjected to mechanical dehydration, and adjustment of pH value is not needed, therefore, the preparation method is convenient; and a supernatant can be recycled, thereby lowering preparation cost, and expanding an application range of the method.