Abstract: The absorbent quality of single down fibers has great impact on the thermal property of down fiber assembly. This paper discussed the primary factors affecting the absorbent quality of down, and tested its absorbent quality under different conditions. Through comparing with other natural fibers and synthetic fibers, the different absorbent qualities from fiber to fiber and the reason of producing this phenomena were expatiated. Finally, an absorbent quality isothermal model of down fibers was developed according to the Hailwood-Horrobin theories in combination with the experimental data.