The accurate measurement of a geomagnetic field is a key technology in geomagnetic navigation. Many magnetic interferential fields on navigation vehicles, such as ferromagnetic parts and electric equipments, will seriously disturb the magnetometer, and thus should be compensated. This paper develops a new magnetic interferential field compensation method using three-axial magnetometer measurements. The method relies on a formulation derived from the difference of the measured geomagnetic magnitude and its true value. The goal is to evaluate the magnetic interferential field parameters first and then compensate the measurements with them. The trust-region method is adopted to evaluate the parameters in the formulation. A simulation is conducted to test the validity of the method. An experiment is designed and implemented on an underwater vehicle. Both simulation and experimental results indicate that vehicle magnetic interferential field can be well compensated with this method.