The laser interference diagnosis system is built independently ,and the phase difference of two laserchannels is cealeulated by analyzing the interferenee image , and then the eleetron density is obtained from thephase difference.The rule of electron density variation with voltage at different positions and the distribution olaxial electron density at different voltages in eylindrical plasma discharge are studied by this diagnostic system.The experimental results show that the eleetron density inereases with the inerease of voltage at differentlocations ,and there is a plateau region of electron density in the increasing process , in which the electrondensity increases slowly with the increase of voltage.There is a density peak in the middle of the dischargetube.When the voltage exceeds a certain threshold,the number of density peaks changes from one to two , andthe electrons accumulate in the area near the anode to a certain extent.