Provided is a plasma diagnosis system using multiple-reciprocating-pass Thompson scattering. The plasma diagnosis system includes: a laser which supplies laser pulses; an optical system configured to make the laser pulse multiple roundtrips, focus the laser pulse to a predetermined position, rotate the plane of polarization by 90 degrees in every completion of the roundtrip; a collection optics which collects lights scattered from the focused region in plasma, ‘first collected scattering’ by the vertical polarization of the laser pulse and ‘second collected scattering’ by the horizontal polarization of the laser pulse; a polychromator which filters the collected lights provided from the collection optics according to spectral characteristics and output the filtered lights; and a computer which measures spectral characteristics of the first and second collected scatterings by using the filtered lights and outputs Thomson scattering signal with the background noise and the background noise without Thomson scattering signal.