Writing a grating by femtosecond coherent pulses, there was an erasure of grating during the reading process, so it is important for research a self-enhancement method to restrain the erasure. In the configuration of femtosecond two waves mixing, two pulses wrote a grating in a LiNbO3:Fe, there was a self-enhancement effect by using a special method that controlled the sequence of femtosecond pulse′s writing: after the two pulse′s writing, single pulse wrote the old grating which made the self-enhancement continue 1 h. At the same time the other self-enhancement effect had been found when the grating was unsaturated and the input two pulses changed to a single pulse, and this self-enhancement continued 2 h. Then the research on the relationship between the writing ultrafast pulse and the writing time had been done, finally the reason analysis had been done with the modulation of femtosecond pulse. This study showed that the diffracted pulse light interacting with writing pulse increased the level of mixing, then a new grating had been written, which boosted up the old one, so that diffraction efficiency increased. The experiment proved that the grating erasure written by femtosecond pulse can be restrained, which was good news for a reading effect of hologram writing.