With the advancement of research and development of weapon systems in China's major scientific projects and defense fields, diamond turning is widely used in the processing of high-performance parts, such as the precision of physical experimental samples and optical parts. Hydrostatic guideways have become the core components of ultraprecision lathes due to their high precision and rigidity. Its geometric error is critical in determining the machining accuracy of ultraprecision lathes. The main factors influencing the geometric errors of hydrostatic guide rails are manufacturing and assembly errors, as well as deformation errors caused by oil pressure. In this research, the fluid-structure coupling simulation model and the bolt preloading mechanical model of the hydrostatic guideway are established to improve the geometric error and accuracy retention of the hydrostatic guideway and discuss the structural deformation and bolt preloading of the hydrostatic guideway caused by the pressure due to oil supply and other factors. The error in a mathematical model to characterize the oil film joint surface under the action of manufacturing, assembly, and oil pressure is proposed using the deformation law of tight structure, and the deformation error transfer model under the action of oil film homogenization is established. The influence mechanism of the oil film bonding surface error of the guide rail pressure plate on the geometric error of the worktable is analyzed. The research shows that the error in the homogenization effect of the pressure oil film of the hydrostatic guideway makes its geometric error much lower than the surface error of its parts, but the pressure oil film cannot completely homogenize the error on the oil film joint surface. The increase in amplitude, linear error PV value, and angle error PV value are proportional as the two types of errors have the same growth rate. The maximum linear and angular geometric errors are 0.22 μm and 1.58 μrad, respectively. This shows that as the surface shape error of the parts increases, the equalization effect of the oil film error gradually weakens. Furthermore, the straightness error of the hydrostatic guideway is more sensitive to the error amplitude of the oil film joint surface, whereas the angle error is more affected by the wavelength and phase difference of the joint surface error function. Finally, the method is used to design the tolerance of hydrostatic guide rail components. When the error amplitude of the oil film bonding surface is within 3 μm, the experimentally measured guide rail straightness error is 0.26 μm/140 mm, and the pitch angle error is 1.72 μrad. The prediction result of the error transfer model for the geometric error of the worktable is close to the experimentally measured result, implying that the model prediction is adequate and the angle error is relatively large. This method can effectively predict the geometric error of the worktable in the designing stage of the guideway. Further, the present research provides a valuable reference for the manufacture and equipment of the hydrostatic guideway of the ultraprecision lathe.