A new vision-based method for measuring linear displacement is proposed. The model of mapping function between the gray value of each pixel and the linear motion displacement is established. The displacement of the linear motion platform is obtained by the numerical method. The Monte Carlo method is used to analyze and verify the feasibility and rationality of the proposed method. In addition, the influence of detection point topology on the measurement accuracy is analyzed. The experimental results show that the standard deviation of the measurement error is 4 μm when the measurement range is 10 mm.