A novel magnetic field sensor was proposed based on the grapefruit optical fiber with the magnetic fluid injected into the six air holes. The sensor utilizes the Mach-Zehnder interference (MZI). The light is transmitted through the single- mode fiber to the fusion splice point and is divided into two parts. When the light passes through the cladding air hole with magnetic fluid, it interferes with the core light, and the change of the interference light is related with the change of the magnetic field. The sensor cavity length is 6 cm. It can be obtained from the experimental results that the magnetic field sensitivity reaches up to 2.243 nm/Oe with the range from 0 to 2.28 Oe. The sensor has the advantages of easy fabrication, easy installation and low cost. The findings provide new ideas for the study of ocean wind electromagnetic fields.