A fast-mounting capacitor is composed of a capacitor (1) and a plug-in sheet (10). A fixed rod (22) extends from the lower end of the shell of the capacitor (1). The fixed rod (22) has a fixed slot (18). The plug-in sheet (10) includes an insulating strip (11) provided above. Plastic elastic clamps (12) and a connection part (13) are connected to the lower part of the insulating strip (11). Fixing clamping strips (17) are provided on the inner wall of the capacitor mounting hole (15). The capacitor can be plugged in conveniently, and can effectively serve as an insulator, therefore it can prevent conducting leakage of electricity to the box of electrical equipment so as to cause accident when abnormal leakage occurs to the capacitor.