The wavelength of fiber surface plasmon resonance(SPR) sensor is usually in the visible light band. In order to make the device work in the optical communication window, a wavelength controllable seawater refractive index sensor based on grating structure is proposed. The sensing area of the sensor is composed of side polished fiber coated with grating structure. The electric field intensity distribution of the cross section of the sensing area, the relationship between different grating structure parameters and SPR sensor wavelength, and the change trend of resonance wavelength with the change of refractive index in the range of seawater refractive index are studied. Finally, the relationship between resonance wavelength and refractive index is analyzed. The sensitivity of the sensor is 5650 nm / RIU, and the resolution can reach 1.77×10-6 RIU, which can realize fast and high sensitivity measurement of seawater refractive index.