Abstract: To accurately predict the straightness of a closed hydrostatic guideway and reveal the coupling mechanism between error transmission and oil film characteristics, a straightness prediction method based on a thin film restrictor is proposed. Through structural analysis, the error transmission paths of 7 sets of parts and 11 joint surfaces are identified, and 6 oil film joint surfaces are identified as key error sources. The straightness data of the guideway surface is fitted by Fourier series. Based on the equivalent rigid body assumption, the balance equation of vertical force and moment is established. The oil film is equivalent to a linear spring model. Combined with the calculation of oil cavity pressure, oil pad bearing capacity and stiffness, the oil film reaction force and average oil film thickness are solved to construct the accuracy prediction model. The experimental results show that after the surface straightness of the key parts of the guideway is controlled below 1 μm, the surface morphology is fitted based on the measured surface data, and the predicted vertical straightness error of the guideway is 0.12 μm/300 mm, which is similar to the measured results and has a basically consistent trend. This has certain guiding significance for the assembly of the guideway and the design of the surface accuracy of key parts.