Long period grating (LPG) is a kind of optical fiber sensing element which is sensitive to the change of refractive index in external environment. Magnetic fluid has the characteristic of changing its refractive index under different magnetic field. In this paper, a novel method for measuring magnetic field of optical fiber is proposed by combining magnetic fluid with LPG and fiber Bragg grating (FBG). Firstly, the LPG is encapsulated in a capillary glass tube filled with magnetic fluid, the shift of the LPG transmission spectrum under different magnetic field is obtained by a optical spectrum analyzer(OSA), the transmission spectrum is shifted by 2.194nm when the range of magnetic field is 0~30mT. On this basis, in order to realize the rapid monitoring of magnetic field, a composite structure of LPG and FBG in series is proposed. The structure is immersed in capillary glass tube filled with magnetic fluid, the external magnetic field information can be detected according to the change of the amplitude of FBG reflection spectrum. When the magnetic field varies, the refractive index of the magnetic fluid will change. Although the FBG is insensitive to the refractive index, its reflection intensity can be modulated due to the shift of the LPG transmission spectrum with the magnetic field. The results show that the maximum sensitivity can reach 0.042dB/mT when the magnetic field range is 0~30mT. This novel fiber grating composite sensing structure can provide a basis for further development of magnetic field monitoring based on intensity modulation.