Aiming at the problem of the high grating inscription scattering of previous microstructured optical fiber, inscription characteristics of a large side-hole microstructured optical fiber grating are analyzed by finite-difference time-domain method and the amalysis result is confirmed by finite element method. The large hole can be filled with fluid of suitable refractive index, the fluid-filled hole acting as a converging lens and the inscribing Gauss beam can focus into the core region, which improves grating inscription efficiency. The simulation results show that the inscription efficiency of the modified structure is 290% of conventional array hole photonic crystal fiber grating and 210% of single model fiber grating, respectively. An ultrahigh inscription efficiency and a relative less grating inscription scattering can be achieved, while the influence of inscription angle deviation is also discussed.