For delivering scalable, reliable and on-demand services, cloud services are becoming more and more prevalent. Due to dynamic and flexibility in cloud environment, Service Level Agreement (SLA) plays a crucial role in guaranteeing the Quality of Service (QoS) level agreed by participants. Though lots of work on SLA management, few of them have adapted well in such an environment. In this study, we present a novel policy-based adaptive approach to solve the problem, and introduce a contract template embedded with policy to handle the changes of service provision and participants' requirement. In this way, we illustrate how the policy enables adaptive SLA management. This approach dynamically perceives alterations of SLAs parameters and adjusts them accordingly. Thus, service providers can maximize the resources and consumers could get the best price. Finally, our proposal is evaluated and its advantages are shown.