In this paper,the positioning theory of single three-component magnetic sensor is studied. The positioning theory model of single three-component magnetic sensor is established,and the theoretical observability of the system is analyzed in detail. Based on the equivalence of the point source magnetic dipole of the magnetic target,the target in the positioning process is regarded as a uniform motion,and the positioning theory model of the single three-component magnetic sensor is derived. Based on the model,the target motion state parameters can be estimated using batch observation data. For the system observability problem,by examining the singularity of the Fisher information matrix,the un-observability of the positioning system of the single vector magnetic sensor is strictly proved. According to the actual application scenario,the observability under partial prior known conditions is derived. The following conclusions are drawn:(1)the single three-component sensor positioning model is unobservable;(2)under partial prior known conditions,the system is observable. Finally,the correctness of the conclusion is verified by simulation experiments,which can provide an important theoretical basis for the tracking of some low dynamic moving targets such as cars and surface ships by single three-component sensors.