[en] The brief development history and general principle of electron cyclotron resonance ion source are presented. The necessary working conditions of ECR ion source for getting highly charged ions are described. The effect of magnetic field upon the confined plasma inside the source is discussed. On the basis of calculations of magnetic mirror and hexapole confinement field, a exactly quantitative concept on the magnetic field is introduced. It is helpful in understanding the picture of the magnetic circuit and magnetic field configuration, as well as the functions of the ferreous parts in the source. Some rooms for improvement of the magnetic circuit and field configuration are found out. All these provide a reliable basis for tuning, operating and partial developing the source