## 1 Introduction  
Magnetic field measurement is of great significance in various fields, such as biomedical, aerospace, military war, the construction of electrical equipment, and so on [1; 2; 3]. The traditional electromagnetic sensor is easy to be damaged or disturbed by the magnetic field [4]. In contrast, optical fiber magnetic field sensors, with the advantages of anti-electromagnetic interference, small size, high stability, corrosion resistance, and remote measurement, are widely studied in the field of magnetic field sensing [5; 6]. Magnetic fluid (MF) is a kind of promising magneto-optical nanomaterial consisting of magnetic particles and base fluid. The MF has no magnetic attraction in static state, and the magnetic particles are evenly distributed in the base fluid in static state. However, a magnetic linkage structure will be gradually formed by the magnetic particles along the direction of the applied magnetic field when the magnetic field is applied, which will cause the effective dielectric constants of MF to change, and then the change in the refractive index (RI) of the magnetic fluid [7; 8; 9; 10; 11]. Hence, the MF has attracted extensive attention for serving as crucial sensing elements of an optical fiber magnetic field sensor owing to the advantages of high sensitivity and good fiber compatibility [12; 13; 14]. The MF-based optical fiber magnetic field sensors mainly include Fabry-Perot (FP) microcavity magnetic field sensor, fiber Bragg grating (FBG) magnetic field sensor, mode interference magnetic field sensor, and surface plasmon resonance (SPR) magnetic field sensor. In 2016, [PERSON] et al. [15] proposed an FP magnetic field sensor with temperature compensation by FBG; the sensitivity is 0.53 nm/mT. In 2018, [PERSON] et al. [16] designed a phase-shifting fiber Bragg grating magnetic field sensor with micro-slit, with sensitivity of 2.42 pm/Oe and the measurement range of 0-120 Oe. In 2019, [PERSON] et al. [17] proposeda tapered optical fiber Mach-Zehnder interferometer (MZI) magnetic field sensor with a sensitivity of 71.98 pm/Oe and 0.11 dB/Oe at 40-120 Oe. In 2019, [PERSON] et al. [18] demonstrated an optical fiber magnetic field sensing system based on SPR with a sensitivity of 303 pm/Gs. In the above literature, the method of gluing-immersing MF was used in preparation of the sensor, which is simple to operate, but generally requires the use of capillary tubes for packing MF, and it also has disadvantages including a high dosage of MF, relatively large volume, and long response time. In addition, because of the use of glue to seal the sensing structure, the MF is prone to be contaminated, which will affect the sensing properties of MF. Compared with the gluing-immersion method, the all-fiber structures prepared by filling-splicing have the advantages of small volume, fast response rate, strong stability, and low cost. According to the research, the smaller the volume of magnetic fluid, the faster the response speed of the MF sensor [19]. In 2017, [PERSON] et al. [20] prepared photonic crystal fiber misplaced fused magnetic field sensors by the method of filling-splicing, and the X-axis sensitivity reached 114.5 pm/mT. In 2018, [PERSON] et al. [21] used multimode fiber and photonic crystal fiber to make a magnetic field sensor by the method of filling-splicing with a sensitivity of 72 pm/Gs. However, the MF inside the porous fibers is carbonized and gasified when fused, which affects the reflectivity of the fiber end face, and eventually the performance of the entire sensor is affected. In order to avoid the influence of the carbonization and gasification of the MF on the performance of the sensor, the air hole filled with the MF should not be used as the light transmission channel. If an optical waveguide can be added to the air hole, it can not only ensure the transmission of light, but also sense the change of liquid refractive index in the air hole, which will be very helpful to improve the sensing performance.