A high-precision six-degree-of-freedom (6-DOF) displacement measurement system with four one-dimensional gratings was investigated to satisfy the displacement measurement requirement of the wafer stage of a high-end immersion photolithography scanner. A 6-DOF system that was capable of measuring the three-degree-of-freedom (3-DOF) translational displacement motions of the wafer stage along the X, Y, and Z directions (X, Y, and Z, respectively), and the 3-DOF angular motions about the x, y, and z-axes (Rx, Ry, and Rz, respectively), was essential for measuring displacements. The optical path structure of this system employed the interference of secondary diffracted beams. The displacement measurement model for recording the displacements of photolithography scanners in real time is constructed, and a simulation verification is performed. The results show that the measurement model errors of X and Y are 0.01 and 0.02 nm, respectively, and the model errors of Rz, Rx, and Ry are 0.37, 1.29, and 0.74 nrad, respectively, without considering the grating and read head installation errors. Furthermore, the measurement model errors of X and Y are 0.06 and 0.09 nm, respectively, and the model errors of Rz, Rx, and Ry are 0.47, 1.36, and 0.78 nrad respectively, considering installation errors. The model error of Z is small and can be ignored. Simulation methods are used to verify the feasibility of the measurement model. The simulation results also show that this model satisfies the requirements for the measurement errors of the wafer stages of photolithography scanners.