The invention discloses a shielded line system for reducing shielding failure exposure arcs of 750kV common-tower double-circuit lines in mountainous areas. The shielded line system comprises a shielded line cross arm, a fixing device, and a shielded line. The shielded line cross arm is fixed to the outer side of a tower body section away from a mountain, and the tower body section is part of a 750kV common-tower double-circuit line tower. The shielded line is fixed on the shielded line cross arm. The tower body section, the shielded line cross arm and the shielded line are integrally disposed below an overhead ground wire, a phase-A conductor, a phase-B conductor and a phase-C conductor of a 750kV common-tower double-circuit line. The shielded line cross arm is longer than the cross arms of the phase-A conductor, the phase-B conductor and the phase-C conductor. For a 750kV transmission line of which the ground inclination angle is larger than 20 degrees in a mountainous area, a shielded line is mounted at the outer side of a tower under the conductors of the transmission line, so as to reduce the exposure arcs of the conductors to thunder and lightning, lower the probability of conductor shielding failure and improve the operation reliability of the transmission line.