Optimization structure of cladding radius and grating periods of long-period fiber grating (LPFG) was proposed, in order to achieve a high sensitivity of sensing the refractive-index of the external medium. Based on the coupled-mode theory of the LPFG, a theoretical analyzation was carried out between the cladding radius and grating period, and surrounding refractive-index, respectively. Effects of the variation of the cladding radius and grating period on the refractive- index sensing were discussed. Both of these parameters were found to be equivalently important to control the refractive-index sensing. Optimizing parameters of the grating are Λ=380 μm and rcl=17 μm in order to achieve a good apply of the LPFG with resonance wavelength in the 1.55 μm common application region. Variation of external medium refractive-index from 1.26 to 1.38 was tested and the achieved refractive-index sensing sensitivity is 0.000 12. Optimization structure of the LPFG achieves a good result.