Sub-Tenancy Architecture (STA), is an extension of Multi-Tenancy Architecture (MTA), allows tenants to offer services for subtenant developers to customize their applications in the SaaS infrastructure . In a STA system, tenants are autonomous decentralized entities who can create subtenants, and grant their resources (including private services and data) to their subtenants. The isolation and sharing relations between parent-child tenants, sibling tenants or non-related tenants are more complicated than those between tenants in MTA. It is important to keep resource private, and at the same time, allow them to be shared, and support application customizations for tenants. This paper provides a formal definition of a new tenant-based access control model based on Administrative Role-Based Access Control (ARBAC) for STA in SaaS. Autonomous Areas (AA) and AA-tree are proposed to describe the autonomy of tenants, including their isolation and sharing relationships. Different resource sharing methods are given out to create and deploy the access control scheme in STA models.