When the desired signal and the interference are in the same direction, in order to improve the communication quality in the ELF (extremely-low-frequency) communication effectively, a kind of interference suppression algorithm based on analog circuits preprocessing and improved GSC (generalized sidelobe cancellation) algorithm was proposed. The magnetic antennas and preamplifier circuits with lower noise were designed, and the magnetic sensor with high sensitivity was produced, which can suppress 50Hz interference and its harmonic components effectively. In view of the weak desired signal in ELF communication, on the basis of the GSC algorithm, several improvements have been made to provide more reference information about interference for the main channel, which can improve the original GSC algorithm′s performance furtherly and address the problem of the traditional algorithm effectively. In order to validate the effectiveness of the proposed algorithm, an experimental platform was set up under the laboratory environment and multiple sets of controlled experiments were performed. Experimental results show that the improved GSC algorithm is better for the improvement of SNR and the noise floor, compared with the original GSC algorithm, regardless of whether the desired signal and the interference are in the same direction.