The physical process of using ECRH to produce sloshing electrons and to form tandem mirror thermal barrier is calculated by means of Fokker Planck kinetic equation.The results show that (1)under certain conditions,the sloshing electron plasma configuration has been formed,the density ratios of which neh(Zr)/neh(0)=2.15 and the potential dip ΔФ=16.5V have also been formed;(2)there are three stages consisting of rapid establishment,gradual decrease and stable equilibrium in the establishment of potiential dip,which result from the coaction of ECRH,collision scattering and trapped ions.