According to the hydraulic supporting structure program of workbench of TH6918 floor boring and milling machine, under unbalanced load condition, the impact of related parameters to hydrostatic round guide between 4 000 mm×5 000 mm workbench and slide was studied. The variation curves of the oil cavity pressure and oil film thickness with unbalanced load position were analyzed. The reasonable position of workpiece was determined. It lays foundation for the further stress analysis and dynamic characteristics research.