Abstract: A SNR estimator based on packet error ratio is proposed，and its statistical property is analyzed for the interference problem in multiple readers environment. An adaptive weighted proportion fairness power allocation algorithm is proposed which can increase the weight of readers in high density area automatically to ensure the read performance for readers in different position. A distributed algorithm is applied to power allocation and its convergence to a local solution is proved. The simulation results show that，the fairness and convergence of the proposed algorithm are superior to the existed allocation scheme.