In order to improve the control precision of overbreak and underbreak of tunnel when using rockdrilling jumbo, the top plate structure on propulsion beam of a kind of threeboom rock drilling jumbo is analyzed. The fault, overlarge distance between guide hole of drilling pipe of top plate and longitudinal horizontal line of the tunnel, exists in the structural design of the original top plate, which leads to difficult control of extrapolation angle of the drill and further would affect the control precision of overbreak and underbreak. [JP2]As a result, optimization, i. e. reducing the extrapolation angle of the drill by shortening the distance between guide hole of drilling pipe of top plate and longitudinal horizontal line of the tunnel, is carried out; so that the overbreak and underbreak of tunnel have been effectively controlled.