The adaptability feature of pervasive computing brings about a series of new challenges which have to be resolved on the software platform layer, such as how to define the architecture of pervasive applications, how to provide common services necessary to the adaptation of upperlayer applications and how to adapt the platform to the multidimensional resourceconstrained environments. Confronted with those challenges, this paper introduces the Autonomic Unit concept, which abstracts the devices and resources in the pervasive computing environments and takes adaptability as an inherent property. Based on this concept, we present the architecture of a selfadaptive pervasive computing software platform UbiStar, and present its design and implementation. The mechanisms to support adaptive software in UbiStar are concluded,and the related works are compared.