A novel power-efficient analog buffer at liquid helium temperature is proposed. The proposed circuit is based on an input stage consisting of two complementary differential pairs to achieve rail-to-rail level tracking. Results of simulation based on SMIC 0.18um CMOS technology show the high driving capability and low quiescent power consumption at cryogenic temperature. Operating at single 1.4 V supply, the circuit could achieve a slew-rate of +51 V/us and -93 V/us for 10 pF capacitive load. The static power of the circuit is only 79uW.