In　order　to　measure　electron　density　caused　by　the　plasma　arc　discharge　in　water,　according　to　the　cause　of　plasma　emission　wavelength　characteristic　plasma　in　water　discharge　by　the　spectrometer　and　using　the　Stark　line　broadening　theory,　the　method　was　put　forward　that　diagnosis　electronic　density　of　water　discharge　plasma　when　the　plasma　belongs　to　the　equilibrium　plasma　state　in　the　center　region.　The　electron　densities　were　calculated　by　the　spectrometer　spectral　lines　using　Stark　line　broadening　theory　theory.　The　calculation　results　show　that:　when　the　electrode　spacing　is　3mm,　the　discharge　voltage　is　1000V,　according　to　the　detected　spectrum　and　the　water　of　spark　discharge　electron　density　of　1.　6199×1021　m-3.　The　proposed　method　can　be　used　for　the　diagnosis　of　water　discharge　plasma　particle　density.　©,　2015,　Editorial　Department　of　Electric　Machines　and　Control.　All　right　reserved.