The invention provides a plastic-clad steel fully-insulated frame assembling capacitor provided with insulating case capacitor units and a mounting method. The capacitor includes an oil tank, wire outgoing sleeves, a capacitor core body and insulating oil; the capacitor core body comprises a fully-insulated frame and capacitor units arranged in the frame; the capacitor units are provided with insulating cases; the capacitor units are fixed in the fully-insulated frame through limiting plates, or the capacitor units can be fixed at corresponding positions of the fully-insulated frame through insulating bolts and by means of mounting flanges with mounting holes machined on the cases of the capacitor units; plastic-clad steel section bars are welded so as to form the main body of the 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 the insulating cases, 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.