Abstract: A training sequence aided frequency-domain channel estimation algorithm based on pilot frequency division multiplexing was proposed for orthogonal frequency division multiplexing (OFDM) based amplify-and-forward (AF) cooperative communication system with multiple relay nodes over frequency-selective fading channels, including least squares (LS) estimation method and low-rank linear minimum mean square error (LMMSE) estimation method. Theoretical analysis showed that the proposed algorithm achieved the entire frequency-domain channel fading coefficients of the multi-node AF cooperative communication system successfully and avoided the superposition of the pilot symbols transmitted by different relay nodes. The proposed algorithm with much better performance needed less pilot symbols and time slots for channel estimation than the traditional algorithm, which improved the transmission capacity and spectrum efficiency of the AF cooperative communication system And the lower complexity and the higher practicality and accuracy of the proposed algorithm were confirmed by the simulation results