Abstract: The brief development history and general principle of electron cyclotronresonance ion source are presented in the paper. The necessary working conditionsof ECR ion source for getting highly charged ions are described. The effect ofmagnetic field upon the confined plasma inside the source is discussed in somedetail. On the basis of the calculations of magnetic mirror and hexapole confine-ment field, a exactly quantitative concept on the magnetic field is introduced.This is helpful in understanding the picture of the magnetic circuit and magneticfield configuration, as well as the functions of the ferreous parts in the source.Some rooms for improvement of the magnetic circuit and field configuration arefound out. All these provide a reliable basis for tuning, operating and partialdeveloping the source.