Nanocellulose fibers (NCFs) aerogels were prepared from poplar alkaline peroxide mechanical pul(APMP) using physical ultrasonication method. As raw materials, the unique mechanical effects of APM cause the fiber folding and loose during the pulping process, which was beneficial to further chemical purification and subsequent treatment for long and entangled NCFs. The obtained NCFs exhibited highe crystallinity (77.8%) compared with that of APMP (72.6%) together with diameters range from 20 t 90 nm and self-assembled to network. The primary thermal degradation of NCFs occurred at 331.5 ◦C. The prepared NCFs network aerogels acted as matrix which can prevent the growth and aggregation of ferro-magnetic CoFe2O4 nanoparticles. The ratio of the CoFe2O4 of magnetic composites increased from 34 wt%to 75 wt%, and the magnetic properties were all increased with increasing the reaction concentration of FeSO4/CoCl2 salt.