Targeting on the ineffectiveness of authorizing mobile clients in the access control system, SDT_RABC--an access control model, which based on supporting space and time dimension was provided. It defined the constraint of the activation area in the spatial environment, spatial cardinality constraint of role activation and separation of duties spatial constraint, a generalized and well-represented control mode was finally set up which enhanced the capacity of spatial safety description to the traditional role-based access control model.