A bias correction method for an electromagnetic test instrument comprises the steps of: acquiring a first field strength measurement value of an HCP coil structure, and a second field strength measurement value of a VCP coil structure (S101); rotating a center point of the VCP coil structure about the y axis by an angle, and then acquiring a third field strength measurement value (S103); calculating, according to the second field strength measurement value and third field strength measurement value, a gradient of a secondary field with respect to a transmitter-receiver separation (S105); and calculating, according to the first field strength measurement value, the second field strength measurement value, and the gradient, a bias error value of an electromagnetic test instrument for the Slingram method (S107). Employing a measured relationship between a secondary field and a gradient of the secondary field with respect to a transmitter-receiver separation by arranging the instrument in different configurations
