Movement parallelism is one of the key parameters of motorized stages, and it can directly affect the performance of the stages. A spot image method for accurately measuring the movement parallelism is proposed. In addition, based on theoretical research and analysis, a measurement system with a precision better than 50 nm is set up. Utilizing this measurement system, movement parallelism of the motorized stage is measured, and the movement parallelism error is 11.66 μm. After optimizing the motorized stage according to the above results, the optimal movement parallelism error can reach 6.22 μm. In order to verify the feasibility of the proposed method, a displacement sensor is applied to re-measure the movement parallelism. The root-mean-square error between the measurement results of the displacement sensor and that of spot image method is lower than 248 nm. In other words, the two measurement results are basically the same.