The flame resistance of kraft paper was greatly modified by an eco-friendly, phosphorus-containing and reactive flame retardant, ammonium phosphite, which was grafted on kraft paper via pad-bake method. The results showed that the limiting oxygen index of treated kraft paper could increase from 19.1 to 48.2%. The results of vertical flammability tests imply that the char length of treated sample decreased from 210 to 45mm. Thermogravimetry analysis showed that treated kraft paper had the lower initial decomposition temperature and more residues than control sample. Thermogravimetry analysis/infrared spectrometry indicated that the flammable volatile species of treated kraft paper reduced obviously compared with that of control sample. Fourier transform infrared spectroscopy suggested that ammonium phosphite was grafted on the cellulose molecules by P-O-C covalent bonds and the flame retardant is reactive in condensed phase. Scanning electron microscopy showed that the modification had little effect on the surface of kraft paper and the residual carbonized frame of treated kraft paper retained the fiber shape after combustion. EDX results showed that the flame retardant introduced a large amount of phosphorus and nitrogen into kraft paper. X-ray diffraction indicated that the modification did not significantly affect the crystal structure of kraft paper. And the strength of kraft papers slightly declined after the modification, but it still remained well.