The road surface at tram grade crossings has borne combined loads of passing vehicles and trams. The load conditions and distresses of this area are complicated and varied, which becomes relatively weak spots on municipal roads. In order to clarify the distress mechanism of this transition pavement at grade crossings, on the basis of field investigation, the statistics and classification of the distresses were carried out, and a 3D finite element model was established to analyze the mechanical response of the road-rail pavement structure, in which combined vehicle and tram loads were conducted. The field investigation results show that the distresses in this transition area are mainly distributed within 30 cm of the track side, and the settlement of pavement near the track is mostly distributed within 0 ~10 mm. The numerical results show that the poor bearing capacity and strength and the disharmony deformation of pavement structure are the main reasons for the failure of the roadrail transition pavement. Focusing on the mechanism of the distress, the countermeasures of laying high modulus concrete under the pavement structure and adding transitional pavement materials between the road and the track are put forward.