Aiming at the problem that the variation of ellipse coefficient is not considered in the widely used magnetic target locating method based on magnetic gradient tensor invariant, which leads to the decrease of location accuracy, a new method of solving unit distance vector and distance vector modulus is proposed.By analyzing the position relationship between magnetic target and measurement system, a new unit distance vector is constructed, and a new distance vector modulus with first-order error and second-order error is constructed by Maclaurin formula and magnetic gradient tensor invariant respectively, to avoid the reduction of positioning accuracy caused by the change of elliptic coefficient.Simulation results show that the proposed method has better positioning accuracy than the previous method, and the distance vector modulus with first-order error and second-order error has almost the same effect on positioning accuracy.The method has high research and application value for the detection of submarine and unexploded ordance.