The invention relates to a 5-hydroxytryptamine receptor agonist, a preparing method thereof and uses of the receptor agonist, particularly (3a[alpha],4[beta],7[beta],7a[alpha])-hexahydro-2-[4[4-(2-pyrimidinyl)-1-(piperazinyl)]-butyl]-4,7-methylene-1H-isoindole-1,3(2H)-dione citrate monohydrate, a preparing method thereof and uses of the monohydrate. The molecular formula of the monohydrate is C<21>H<29>N5O2.C6H8O<7>.H2O. The monohydrate has advantages of high stability and good solubility, thus facilitating product quality control, increasing dissolution rates of preparations, and facilitating medicine absorption and utilization. The monohydrate can be prepared into various dosage forms conveniently, and is convenient to process, store and convey. The preparing method has advantages of few steps, simple and convenient operation, a high product yield, high purity, and the like and is particularly suitable for industrial application.