As the matrix of sisal fiber, thermoplastic tapioca starch(TPS) was prepared with the mixed plasticizer, formamide and urea (mass ratio 2:1). X-ray diffractograms showed that with increasing fiber content(below 30phr), cellulose crystallinity at 22.5° gradually got stronger but starch crystallinity at 15.3°, 17.1°, 18.2°, 23.5° already disappeared, showed that this matrix still restrained the retrogradation of starch. SEM micrographs showed good dispersion and adhesion between starch and fiber. Studies in the dependence of mechanical properties of reinforced TPS on the fiber content from 0 to 30phr, the initial tensile strength was quadrupled up to the maximum 21.83MPa at 20phr fiber content, while the elongation at break was reduced from 72% to 0.44%. TG mass loss curves showed that thermal stability of this composites had great improved under 500°C.