In the presented work, a series of the phosphate amphoteric cassava starch were synthesized by a two-step semi-dry process with 3-chloro-2-hydroxypropyltrimethyl ammonium chloride (CTA) as the cationic reagent and phosphate mixture as the anionic reagent. The structure of the amphoteric starch was characterized by FTIR and XRD. The reinforcing effects of the amphoteric starch on the secondary fiber were also studied. The results indicated that the reaction occurred not only in the amorphous region, but also in its crystalline region. When the cationic starch reacted with different amount of phosphates, the degree of cation substitution (DSc) of products was decreased with the dosage of the phosphates. When cationic starch with different DSc reacted with the same amount of phosphates, the degree of anion substitution (DSa) of products was decreased with the DSc. All the amphoteric starch showed reinforcing effect on the secondary fiber. The No. 7 sample showed better reinforcing effects on paper strength due to its higher total degree of substitution (0.095), proper DSa/DSc ratio (DSc = 0.067, DSa = 0.028) and interior modification of the starch granule. Compared with the control sample, the secondary fiber paper with 1.0 wt % (relative to dried pulp) of 7# modified starch showed 23.5%, 20.3% and 29.4% increases in tensile index, tearing index, and burst index, respectively.