Ram is the key part influencing the machining accuracy of gantry type boring and milling machine. Aiming at actual problems in cutting, the heat sources causing thermal deformation of ram were found out. The heat generation rate of bearings was calculated, and the mathematic model of thermal deformation was set up. The thermal performance on the ram was analyzed, and the structure of bearings of the spindle unit was improved. The result shows that the highest heat point of 64.13 ℃ is at the bearing support part, and the biggest thermal deformation part which is 38.9 μm, it is at the spindle bearing support end. Using the cooling sleeve unit and rear bearing free support structure can reduce the thermal deformation of spindle and ram, and so to improve the machining accuracy of gantry type boring and milling machine.