The same grid user may have another identity in different domains, and same to grid resource which have been restored and maintained in different domains. It is a challenging task for executing grid jobs successfully to make the same user have the consistent privileges in grid. However, distributed characteristic of grid makes it difficult to define and maintain global privileges consistency. And dynamic characteristic of grid seriously threatens global privileges consistency defined in grid access control. This paper proposes a dynamic access control model embodying access policies and its dynamic negotiating. The model supports to define global privileges consistency and automatically resume consistency when conflict has occurred. We also describe a prototype implementation of this model and its performance.