A cotton-free electronic cigarette preventing liquid spill, comprising a mouthpiece assembly, a battery control assembly and a vaporizer assembly, the mouthpiece assembly and the battery control assembly being respectively connected to two ends of the vaporizer assembly, the vaporizer assembly comprises: an outer sleeve, the outer sleeve being a pipe having an inner diameter larger than an outer diameter of a liquid storage cup of the cotton-free electronic cigarette, the liquid storage cup, the liquid storage cup being a cylinder having a segmental-circular cross section formed by an exterior longitudinal flat surface, and a vaporizer unit, the vaporizer unit comprising an electric heating assembly and a power connecting assembly; characterizd in that: the vaporizer assembly further comprises a heat insulation sleeve having a tubular structure, an exterior surface of an outlet end of the liquid storage cup is provided with a stepped seating, the heat insulation sleeve is configured to be fitted on the exterior surface of the outlet end of the liquid storage cup; the liquid storage cup stores cigarette liquid therein, an opening of the liquid storage cup is provided with a liquid percolation piece and a filter piece; the vaporizer unit is placed inside the heat insulation sleeve; the exterior longitudinal flat surface of the liquid storage cup forms an air passage when the liquid storage cup is fitted into the heat insulation sleeve; the liquid storage cup is sheathed with the outer sleeve, an inner wall of the outer sleeve and an exterior arc surface of the liquid storage cup forms an arc-shaped slit, the arc-shaped slit being used for preventing spill of residual condensed cigarette liquid; an inner wall of the outer sleeve and the exterior longitudinal flat surface of the liquid storage cup forms a vapor flow passage, the vapor flow passage is interconnected to the arc-shaped slit; the mouthpiece assembly is fixed to one end of the outer sleeve to seal an outlet of the arc-shaped slit, such that an inhalation hole of the mouthpiece assembly is interconnected to the vapor flow passage. an outer sleeve, the outer sleeve being a pipe having an inner diameter larger than an outer diameter of a liquid storage cup of the cotton-free electronic cigarette, the liquid storage cup, the liquid storage cup being a cylinder having a segmental-circular cross section formed by an exterior longitudinal flat surface, and a vaporizer unit, the vaporizer unit comprising an electric heating assembly and a power connecting assembly; the vaporizer assembly further comprises a heat insulation sleeve having a tubular structure, an exterior surface of an outlet end of the liquid storage cup is provided with a stepped seating, the heat insulation sleeve is configured to be fitted on the exterior surface of the outlet end of the liquid storage cup; the liquid storage cup stores cigarette liquid therein, an opening of the liquid storage cup is provided with a liquid percolation piece and a filter piece; the vaporizer unit is placed inside the heat insulation sleeve; the exterior longitudinal flat surface of the liquid storage cup forms an air passage when the liquid storage cup is fitted into the heat insulation sleeve; the liquid storage cup is sheathed with the outer sleeve, an inner wall of the outer sleeve and an exterior arc surface of the liquid storage cup forms an arc-shaped slit, the arc-shaped slit being used for preventing spill of residual condensed cigarette liquid; an inner wall of the outer sleeve and the exterior longitudinal flat surface of the liquid storage cup forms a vapor flow passage, the vapor flow passage is interconnected to the arc-shaped slit; the mouthpiece assembly is fixed to one end of the outer sleeve to seal an outlet of the arc-shaped slit, such that an inhalation hole of the mouthpiece assembly is interconnected to the vapor flow passage.