An interferometric microfiber magnetic field sensor composed of a microfiber interferometer and a TbDyFe magnetostrictive rod is proposed. The microfiber interferometer was formed by tapering a single-mode fiber and placed in lateral contact with a TbDyFe magnetostrictive rod. Under the action of a magnetic field, the axial strain of the magnetostrictive rod and the microfiber interferometer causes a wavelength shift of the interference spectrum, which forms a wavelength coding fiber magnetic field sensor. The experimental results show that for the microfiber interferometers with the same strain characteristics, the smaller the diameter of the magnetostrictive rod is, the higher the magnetic field sensitivity is. The sensitivity of the fiber magnetic field sensor composed of TbDyFe magnetostrictive rod with a diameter of 2 mm can reach up to 0.178 nm/mT. The sensor has the advantages of simple structure, easy fabrication, low cost, and fast response. It can realize highly sensitive detection of weak magnetic field.