Two models are developed for ECR plasma source stream in the diverging magnetic field. In the Analytic Model, the plasma equation for a collisionless plasma is formulated and solved analytically in the source region and sourceless region. In the Collisional Model, elastic scattering and charge exchange collisions between ions and neutrals in the downstream region are treated by Monte-Carlo method. Through two models, the effects of the magnetic field and neutral gas pressure on the plasma potential and ion energy distribution are studied.