In order to solve the problem of navigation accuracy decline caused by GPS signal losing lock in a short time, a vehicle-mounted Hall odometer is designed and constructed based on the principle and characteristics of Hall switch. According to the parameters of the target gear and the simulation optimization results of the magnetic circuit design, the shape and magnetic flux of the magnet steel are optimized, and the size of the magnet is determined. Based on the Hall effect and induction theory and technology, the magnetic field perturbation is simulated by using the magnetic field finite element software. The optimization results show that the magnetic field disturbance can be effectively identified in the range of less than 4 mm, which improves the accuracy of vehicle mileage calculation and greatly improves the anti-electromagnetic interference and anti-temperature drift ability of Hall odometer.