Abstract: The dynamic sheath model and the analytic expression of the vacuum arc plasma for mixed ion beam are presented based on the collision less model. The variations of the sheath width and electric-field strength as the function of the ion density and substrate bias voltage are calculated respectively for H-Ti mixed ion beam. The phenomena of sheath breakdown and ion beam defocusing, which usually arise in the practical application, are analyzed. It is found that the stable operating region for the mixed ion beam extremely depends on the ion density and substrate bias voltage, and reducing the ion density and increasing the substrate bias voltage can expand the stable operating region.