The utility model provides an automatic capping device of torpedo jar, include: the cantilever telescoping device, the top draw the device and with torpedo jar mouth assorted cover, the cantilever telescoping device includes: cantilever seat and through the vice cantilever that is connected with the cantilever seat of rectilinear movement to and the flexible power drive device of driving cantilever, the free end at the cantilever is installed to the cover, and can arrive the top of torpedo jar mouth when the cantilever stretches out, and the cantilever seat articulates on torpedo jar support body or automobile body through first articulated portion, and the top draws the base bottom of device articulated mutually with torpedo jar support body or automobile body, and the top is drawn the top of device to draw and is articulated mutually between main shaft upper end and the cantilever seat, the utility model discloses a free end at the cantilever is installed to the cover, and the cantilever need not bear the overturning moment along the width direction on, the cover the installation with take off the in -process and only need all stretch out the cantilever and withdraw along the straight line, on can be the midplane of cantilever setting at torpedo car width direction when installing, can effectively avoid the danger of turning on one's side or derailing like this on the bend.