TK6920B CNC floor type boring and milling machine is a type of heavy duty machine tool, and it is easy to generate flexural deformation during the extension of its ram due to its weight and the gravitational off-center of the ram and the headstock, which affects the machining accuracy during processing. In order to solve this problem, the prestress flexural deflection machining method and hydraulic rod compensation method were proposed. Ansys WB software was applied to analyze and predict the ram flexural deformation and compensation effect so that it can cut down experiment cost. By comparing the measured flexural deformation of the ram with the finite element method (FEM) simulation results, it shows that deformation error is controlled within 5 μm, the FEM simulation results are reliable and the total ram deformation is about 20 μm. It is proved that proposed compensation methods are also effective and can meet the requirements of machining.