The invention discloses an under-actuated track type vehicle wheel mechanism. The adaptive capacity of an existing track type vehicle wheel mechanism to complex varied terrains and unstructured terrains with large slope change needs to be strengthened. The under-actuated track type vehicle wheel mechanism comprises a drive assembly, a driving wheel assembly, a driven wheel assembly, a swing arm assembly and a track. The drive assembly drives the driving wheel assembly, the driving wheel assembly is connected with the driven wheel assembly through the track, and thus the driven wheel assembly is driven to rotate. The driving wheel assembly and the driven wheel assembly are connected through the swing arm assembly, so that the driven wheel assembly is driven to swing. The driving wheel assembly comprises a first speed reducer, a second speed reducer, a spindle, a driving wheel, a second outer meshing gear, a first outer meshing gear, a planet arm, a second planet carrier, a hollow shaft, a second gear, an output shaft, a first planet carrier, a first gear and a spindle bevel gear. According to the under-actuated track type vehicle wheel mechanism, a passive type conversion traveling mode can be realized according to practical terrain conditions, and therefore the adaptive capacity of a mobile robot to the unstructured terrains can be effectively strengthened.