This project provides a fabrication method of water soluble fluorescent carbon quantum dots from fresh lemon juice. Technical features of the method are as follows: the precursor made by fresh lemon juice and alcohol mixed in proportion was constant temperature heated for 150 min at about 120℃, and centrifuged at 10000 r/min speed for 15 min, and then dialyzed, fluorescent carbon quantum dot aqueous solution can be obtained. The water-soluble carbon quantum dots prepared with the method of low cost and simple process show strong fluorescence, low cytotoxicity and good biocompatibility. The carbon quantum dots would be applicable to the luminous materials and display devices, green lighting, solar battery, super capacitors, sterilization, living cell markers, biological imaging and other fields.