A new type of fiber Bragg grating voltage sensor which is based on joule heat was designed and used to precisely monitor small voltage.In the experiment, a layer of nickel was plated around the grating by vacuum evaporation method and the grating was hermetically packaged in copper tube which is full of aerogel.The center of reflection spectrum of fiber Bragg grating drifts as the fiber grating heated due to the heat generated in nickel layer when the current goes through.Thus the transformation from voltage change to centre wavelength drift could be obtained.The experiment results show that the direct current voltage sensitivity after packaged is up to 520 pm/V2, and it can be used in the low frequency alternating current sensing.This system owns the advantages of simple structure, high accuracy and easy to be implemented.