Abstract: Compared with traditional wireless communications, deep space communications are challenged with difficult problems of extremely low Signal-to-Noise Ratio (SNR), high transmission delay and discontinuous reception of data which are mainly caused by large transmission loss, limited energy and changeful trajectory of the space probe, etc. Faced with these problems, the researches on several key techniques at extremely low SNR in deep space communications are introduced. And the techniques include deep space antenna arrays, coding and modulation, joint demodulation and decoding, fountain coding and the autonomous radio and so on to help solve the above problems. Finally, the development trends and related applications of them are prospected.