A graphene oxide-functionalized long period fiber grating (GO-LPFG)-based fiber optic sensor is proposed. The surface of the LPFG was hydroxylated by sodium hydroxide solution, and GO was fixed on the grating surface by hydrogen bonding to form a GO modification LPFG sensor. The responses of the GO-LPFG to external refractive index and temperature were studied experimentally. The experimental results show that the average refractive index sensitivity of the GO-LPFG is 1.09 times higher than that of uncoated LPFG, and the temperature sensitivity slightly declines. With the decrease of grating diameter, the average refractive index sensitivity of GO-LPFG was further improved. The average wavelength and coupling intensity refractive index sensitivities of the GO-LPFG with a diameter of 108 μm in the refractive index range of 1.333-1.448 are ~38.99 nm/RIU and ~57.33 dB/RIU, respectively. These increased by 1.45, 2.17 and 3.80, 3.42 times, respectively, compared with those of bare LPFG and GO-LPFG with diameter of 125 μm. The proposed GO-LPFG sensor has potential applications in detecting various viral antigen proteins with high molecular weight and biological pathogens in biofields.