The present invention provides a fully-insulated frame assembling capacitor provided with insulating case capacitor units and a mounting method. The capacitor comprises an oil tank, wire outgoing bushings, a capacitor core body and insulating oil. The capacitor is characterized in that the capacitor core body includes a fully-insulated frame and a plurality of capacitor units disposed 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; the fully-insulated frame is made of a glass fiber composite material; the frame is realized by means of the connection of insulating connection sheets and insulating bolts; the whole frame is fully insulated; and capacitor oil is adopted asthe insulating oil. 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 entirecapacitor device can be reduced.