Security access is one of the key concerns for wireless sensor networks (WSNs). The secure authentication protocols of the most current security access schemes are complex. Moreover, the access control models of existing schemes have no concept of attribute mutability and can not perform continuous access decisions. In this paper, we propose a new security access scheme that deal with the requirements of low-complexity authentication and continuous access decision. The authentication protocol and access control mechanism is designed based on security token and usage control (UCON) respectively. An instance of UCON is implemented. The implementation results support the feasibility of using UCON in WSNs. The authentication protocol in our scheme presents several advantages including the low expenses in calculation, storage and communication. Moreover, our scheme also can perform access control with attribute mutability and decision continuity.