To achieve high-sensitivity detection of direct current (DC) magnetic fields in electrical equipment and overcome the limitations of conventional magnetic field sensors, such as poor electromagnetic interference resistance, large size, and complex structure, a magnetic field sensor based on dual-tapered fiber was proposed. The optical field energy distribution within the dual-tapered fiber was simulated, and the principles of dual-mode interference sensing and magnetic fluid characteristics were analyzed. The dual-tapered fiber was fabricated using an electrode discharge heating tapering method, and the magnetic field sensor was developed by filling it with magnetic fluid. A magnetic field detection experimental platform was established, and the sensor's response to magnetic fields as well as its repeatability were experimentally validated. The results demonstrate that the sensor exhibits a linear response within a magnetic flux density range from 0 mT to 10 mT, with a linearity of 0.998, magnetic field sensitivity of -173.96 pm/mT, an error within± 0.3 mT, exhibiting excellent repeatability. The sensor is shown to meet the mT-level magnetic field detection requirements for power transmission and transformation equipment.