An important challenge regarding peer's trust valuation in P2P systems is how to cope with strategically altering behaviors of malicious peers efficiently. However, the trust metrics employed by the existing systems do not provide adequate support to track this dynamics and impact dynamic adaptability of trust valuation. In this paper, we propose an adaptive peer-to-peer dynamic trust metric. Considering the dynamics of trust which relates to the evolution of trust in time and in the face of new experiences, we introduce an adaptive forgetting scheme, where the forgetting factor is adjusted in time to reflect quick changes of peers? behavior. Thus, the trust evaluation has better adaptability to the dynamic of trust. Theoretic analysis and simulation show that, comparing with existing trust metric, our metric has advantages of adaptability on modeling dynamic trust and is highly effective in countering malicious peers regarding strategic altering behavior.