To solve the difficulty of generating magnetic maps in magnetic guidance system and improve the poor applicability of the existing methods on multi-intersection mapping, a method applying the feature of magnetic segment is proposed in this paper. First, IMU, odometer, and magnetic ruler were used to collect the road magnets data, and then the data were divided into magnetic segments based on the signal magnetic segment. After that the features of the magnetic segments were extracted from the sensors data, they were matched and loop closure constraints were established between the segments. Second, by using the improved graph-based optimization algorithm, the influence of error constraints was reduced. Finally, the corrected magnets were obtained from the optimized graph and the magnetic map could be generated. Based on the experimental results of a real road, it can be concluded that the method is simple to operate and the obtained magnetic map is accurate with high global consistency. The guidance experiment also verified the usability of the method. Therefore, this method has a wide applicability on multi-intersection mapping and it can effectively generate magnetic maps.