The experiment of one-time writing long period fiber grating is performed. It not only improves the efficiency of fiber writing process, but also solves the problem of multisession mismatching. So it has important research value. From the experiment, we find that effective refractive index periodical modulation and appropriate physical deformation occur in the fiber core and cladding under appropriate pressure and radiation, and the LPFG is one-time written successfully using high-frequency CO2 laser in single-mode fiber. Experimental results show that the grating transmission spectrum features in one-time writing process are better than that in multiple writing process.