The invention provides a method for determining the number of shielding grids of a power transmission line. The method comprises the following steps: acquiring electrical parameters such as live-linepower transmission line parameters, ground wire parameters, soil resistivity parameters along a power transmission line, live-line operation power transmission line operation data and the number of shielding grids; a power transmission line simulation model and a shielding net model are established based on CDEGS software, electric field intensity values below a power transmission line when the number of shielding nets is different are calculated, and the optimal shielding net number is determined according to the field intensity values. According to the method, the optimal grid number is determined, the shielding net is arranged for the power transmission line, and a powerful guarantee is provided for normal life of residents below the power transmission line.