The work aims to improve the kinematic accuracy of the trajectory output point of the M-shaped prefabricated bag opening folding mechanism, and optimize the kinematic accuracy reliability of the opening folding mechanism. Based on kinematic analysis, a position error model of the opening folding mechanism considering rod length errors was established according to the loop increment method. Then, reliability analysis and Monte Carlo verification were performed on the trajectory output points, and key error influencing factors were determined through sensitivity analysis. Finally, reliability optimization of motion accuracy was performed. The results showed that the reliability model established could effectively reflect the impact of rod length errors on the kinematic accuracy of the mechanism. The reliability of the x-component trajectory was increased from 82.5% to 92.91%, and the reliability of the y-component trajectory was increased from 65.34% to 89%. In summary, the reliability optimization can make the movement accuracy of the opening folding mechanism meet the design requirements.