In this paper a physical layer authentication method is proposed to suit to time-variant wireless channels by taking advantage of the varied mutually orthogonal basis functions and the constant basis coefficients to approximate the state of separable paths, in which each sampling values of separable paths is related within a block transmission time in time-variant channels. Simulation experiments in orthogonal frequency division multiplexing (OFDM) system proves the validity of the novel authentication method. The novel method makes 2-4dB improvement compared to the physical-layer authentication using conventional Least Square (LS) channel sounding.