The invention provides a plastic-clad steel fully-insulated frame assembling capacitor provided with unshelled capacitor units and a mounting method. The capacitor includes an oil tank, wire outgoingsleeves, a capacitor core body and insulating oil. The capacitor is characterized in that the capacitor core body comprises a fully-insulated frame and capacitor units arranged in the frame; the capacitor units are unshelled capacitor units; the capacitor units are fixed in the fully-insulated frame through limiting plates; plastic-clad steel section bars are welded so as to form the main body ofthe fully-insulated frame; section bar welding parts are treated by means of glass fiber cloth winding, insulating bolt connection parts are sealed and insulated by means of insulating resin encapsulation, and therefore, the full insulation of the frame can be realized. The capacitor units are provided with no small shells; the capacitor units are fixed in the frame through the limiting plates soas to form the capacitor core body; and therefore, the size and weight of each capacitor unit can be further reduced, and the size and weight of the entire capacitor device can be reduced.