The invention discloses an online oil injection method for a bearing of an outdoor high pressure isolating switch base, wherein an online oil injection device and an online oil injection method are utilized. The online oil injection device comprises a forced filling oil cup and a grease gun, wherein the forced filling oil cup is arranged on the high pressure isolating switch base provided with a screw hole; the forced filling oil cup is connected with the screw hole; and the forced filling oil cup matched with the diameter of the screw hole is installed in the screw hole by utilizing the online oil injection device. In order to ensure the sealing performance, after a waterproof belt is wound outside the screw thread of the forced filling oil cup, the forced filling oil cup is installed in the screw hole. Grease is injected into the bearing in the base by utilizing the grease gun in the operation process of a high pressure isolating switch. As a sufficient safe distance between the high pressure isolating switch base and the electrified part of the high pressure isolating switch is reserved, online oil injection can be performed without power cut, therefore, the method is safe and convenient and can effectively ensure the stable operation of the outdoor high pressure isolating switch.