A novel fiber-optic current sensor based on a nested fiber grating is proposed. Long period grating fabricated by two-exposure method and Bragg grating are nested to form fiber grating, and it’s used as sensing element. Surface of the nested fiber grating is sputtered with a layer of high resistance Ni-Fe alloy, which is used as heating electrode layer. When the current passes through, the grating region is heated, which leads to center wavelength of grating spectrum shifting. High sensitivity current measurement is realized. Experimental results show that the current response sensitivity of the designed optical fiber current sensor reaches 7.2×10-2 nm/mA. It has the advantages of simple structure, small size, and has wide application value in the field of high sensitivity optical fiber current detection.