The invention relates to a method for measuring and calculating magnetic field intensity near a high-voltage power transmission line on a mountain slope, and belongs to the field of field intensity measurement and calculation of electric magnetic fields. In the kth span, a standard coordinate system XYZ is established in a plane of the high-voltage power transmission line; in the kth span, a coordinate system X'Y'Z' is established in the plane of the high-voltage power transmission line; a catenary equation model of the high-voltage power transmission line is obtained; a transformation matrix model of the coordinate system X'Y'Z' and the standard coordinate system XYZ is obtained; through a Biot-Savart law, eddy is generated based on a current of the high-voltage power transmission line and a poor conductor of the ground; simplification is performed; the magnetic field intensity, defined in the specification, of any point, in the standard coordinate system XYZ, of the high-voltage power transmission line in the kth span is obtained; the magnetic field intensities in continuous three spans are superposed; and the magnetic field intensity, defined in the specification, of any point, in space, of the high-voltage power transmission line in the kth span is obtained. Compared to the prior art, the method considers an arc sag factor of the high-voltage power transmission line and can realize measurement and calculation of the magnetic field intensity of any point near high-voltage power transmission lines in different planes.