The invention discloses a preparing method of a cesium 
ion adsorbing agent. The preparing method comprises the steps that firstly, carboxyl ferroferric 
oxide is prepared; secondly, carboxyl ferroferric 
oxide and a condensing agent are mixed, and a first mixture is obtained; thirdly, the first mixture is put into a 
solvent and stirred, an 
amination crown ether derivative is then added into the 
solvent, a reaction is carried out at the temperature 
ranging from 60 DEG C to 80 DEG C for at least 2 hours, and a second mixture is obtained, wherein the 
solvent is used for dissolving the 
amination crown ether derivative and the condensing agent; fourthly, the second mixture is subjected to 
solid and liquid separation, a first 
solid phase is obtained, cleaned and dried, and the cesium 
ion adsorbing agent is obtained. The invention further discloses the cesium 
ion adsorbing agent prepared through the method. The cesium ion adsorbing agent has high cesium 
ion adsorption selectivity and 
magnetism, and stable adsorption performance is maintained in the recycle use of adsorption and 
desorption. The cesium ion adsorbing agent can be applied to 
adsorption separation of 
cesium ions form radioactive nuclear waste, a polyion competing 
system and the like.