Ag nanoparticles are prepared by microwave synthesizing method and absorbed on the glass slide by self-assembling technology. Thus, the localized surface plasmon resonant (LSPR) sensor is developed. UV-vis absorption spectrum shows that its LSPR peak is located at 428nm in pure water. The LSPR peak takes red shift when the refractive index of surrounding solution increases, and the sensitivity can arrive at (173±6) nm/RIU. After annealing at 350 ℃, the shape of Ag nanoparticles on the glass slide is changed. The LSPR peak shifts about 65 nm towards longer wavelength, and the sensitivity decreases with 20%. Theoretical analysis indicates that shape and substrate take important effect on the response of wavelength and the sensitivity for the refractive index sensor of Ag nanoparticles.