The invention discloses a magnetic field differential method-based transformer substation grounding grid topology structure drawing method. In the method disclosed in the invention, a pair of grounding grid down lead wires are chosen, one grounding grid down lead wire is used as a current injection point while the other grounding grid down lead wire is used as a current flow-out point, a constantcurrent I is injected into a grounding grid, a conductor branch circuit which intersects with a current flow-out point of the grounding grid down lead wire is drawn, a space rectangular coordinate system is built, branch circuit nodes along the drawn conductor branch circuit are determined, and a network model of a grounding grid can be preliminarily drawn; positions of other regular conductor branch circuits are determined according to the drawn conductor branch circuit, the The method is used for search along a line, quantities of sampling points can be reduced greatly, and branch circuit drawing processes are simple and high in efficiency.