The invention discloses an installation device of an electrode cap of a spot welding machine. The device comprises a T-shaped rod, an n-shaped rod, a connecting pin, a lock pin and a handle. The T-shaped rod and the n-shaped rod are respectively provided with countersinks which are matched with the electrode cap in shape. The cross section of the connecting pin is a variable section, and the connecting pin is provided with a plurality of installation holes. The lock pin and the handle are arranged in the installation holes at both ends of the connecting pin. Through holes are arranged in the side face at the small end of the T-shaped rod and two side faces of the n-shaped rod. The through holes are matched with the small end of the connecting pin in shape. The distance from the central lines of the through holes to the large end face of the T-shaped rod is equal to that from the opened end of the n-shaped rod to a corresponding end face. The cross section at the small end of the T-shaped rod is matched with that of an n-shaped groove of the n-shaped rod in shape. The small end of the T-shaped rod threads through the n-shaped groove of the n-shaped rod and is connected with the groove through the connecting pin. The installation device provided by the invention has the beneficial technical effects that the end part of the electrode cap is prevented from being damaged, the service life of the electrode cap is prolonged, and automatic centering of the electrode caps up and down is ensured, so that the spot welding quality is improved.