Abstract: The determination of the optimum draggle distance between the sensor and the surveying boat is an important task in marine magnetic survey.It will greatly influence the surveying precision and working efficient.The surveying boat was seemed as a magnetic dipole in this paper,and the magnetization model of the boat and the formula that calculate the magnetic field strength behind it was deduced.The theoretical method which determine the optimum draggle distance was given out.The applicability of the method was validated with the practical test.