In this paper, the principle of interferoinetry for determining electron density Of plasma is presented. The structure of 7-ohannel interferometer and its measurement results on HT-6M TOKAMAK are described. The light source used in the interferometer is a o. w. glow HON laser with cavity length of 3.4M and power output about 100 mW at 337μm. The averaged electron density of 7 probing chords can be given simultaneously. The detectable sensitivity its 1/15 frige with temporal resolution of 0.lms. Through asymmetrical Abel inversion from the line integrals of phase shift of 7 chords, the electron density profile at different time or three dimensional diagram of space-time distrirbutions of electron density can be obtained.