Provided is an undervoltage tripping device. The undervoltage tripping device comprises a movable iron core, a static iron core and a coil, wherein a through hole with a height of B is formed in a large side face of a shell base which is installed together with an outer side face of a matched alternating-current motor starter or matched alternating-current motor circuit breaker in a tightly attaching mode. The undervoltage tripping device further comprises a transmission rod which is matched with a lever to work, wherein the lever is in linkage with the movable iron core, and the transmission rod is in linkage with the lever. A rod body of the transmission rod is movably installed on the shell base in a linear mode through a transmission mechanism. A touch arm, formed on the rod body, of the transmission rod comprises a combination portion and a touch head, wherein the combination portion and the touch head are integrally formed. The touch head extends out of the shell base through the through hole, and drives an operation mechanism of the matched alternating-current motor starter or an operation mechanism of the matched alternating-current motor circuit breaker to carry out movement. Notches which allow increase of the height of the coil are formed in the position of connection between the touch head and the combination portion, a separation distance is kept between the notches, and due to the arrangement of the separation distance, the transmission rod can achieve vertical movement without interference of the coil, the boundary dimension of the undervoltage tripping device cannot be increased at when the height of the coil is increased, the heat dissipation performance of the undervoltage tripping device is effectively improved, and heat transmission influence from the alternating-current motor circuit breaker is reduced.