The invention discloses a preparation method of a monolayer MoS2 nano-sheet solution. The preparation method provided by the invention comprises the following steps of weighing MoS2 powder, putting the MoS2 powder into a pyrrolidone solvent, carrying out magnetic stirring of the mixture, transferring the stirred mixture to a hydrothermal reactor, carrying out hydrothermal pre-treatment, adding a surfactant into the mixture subjected to hydrothermal pre-treatment, carrying out ultrasonic stripping by an ultrasonic destrustor, carrying out centrifugation treatment, and collecting a supernatant liquor, wherein the supernatant liquor is the monolayer MoS2 nano-sheet solution. Compared with the prior art for preparing a monolayer MoS2 nano-sheet solution by a reaction of MoS2 and alkyl lithium and ultrasonic stripping, the preparation method provided by the invention has the advantages of the adopted organic solvent is not sensitive to the environment; a whole preparation period is short; and the obtained monolayer MoS2 nano-sheets have better dispersibility and stability and a lower cost.