The invention relates to an easy-to-install isolating switch mechanical locking device and a working method. Configured line side grounding disconnecting links for outlet disconnecting links of 220kV lines of part of substations and main knives cannot meet the related requirements of anti-accident locking execution of the isolating switch. The easy-to-install isolating switch mechanical locking device comprises a primary locking plate (1) and a secondary locking plate (5), wherein the primary locking plate (1) comprises a connecting disc (2) and a first lock disc (4); the connecting disc is provided with three U-shaped holes (3); the secondary locking plate (5) comprises a first connecting shaft (6), a second connecting shaft (7) and a second lock disc (18); the first lock disc and the second lock disc are matched and connected; the first connecting shaft is connected with a shaft sleeve (8); the second connecting shaft is connected with a transmission shaft (10) of a universal shaft (9); the universal shaft comprises the transmission shaft and a connecting rod (11); and the connecting rod of the universal shaft is connected with a transmission rod (13). The easy-to-install isolating switch mechanical locking device is used for an electric power overhaul device.