Monte Carlo method is used to simulate such a complete course, in which the ions pass through the neutral region, the sheath, and the shield region of a virtual cathode formed by secondary electrons near the workpiece surface, finally come to be absorbed by the surface of the workpiece with negative bias in ECR microwave plasma.Discussion covers the connection problem of neutral region and sheath at the sheath edge.The non linear Poisson equation is solved with "trial" method, and results in smooth and self consistent spectrum of potential and distributions of ion energy and angle at the surface of workpiece.