Abstract: In order to fulfill the spectrum sensing in low SNR environment, the spectrum sensing method, free of noise uncertainty, based on LMS is proposed. In this paper, LMS algorithm is applied to estimate the amplitude of transmitted signal in real time, and the estimation is employed as the test statistic to detect the existence of primary user. Both the theoretical and simulation results show that the proposed method has better performance for weak signal detection, which also greatly outperforms the classical energy detection method and effectively overcome the influences of noise uncertainty by estimating the noise variance.