Abstract: A robust mathematical design model based on the response surface method was established for steering trapezoid mechanism. In the model, movement precision during steering course was chosen as the objective function; kingpins distance, axles distance, trapezoid mechanism's bottom angle and steering arm length were defined as the design variables; minimum transmission angle and maximum angle error were considered as the restraint condition. Steering mechanism of a truck was optimum designed by adopting the method of Monte Carlo optimization design with distribution parameters. The results of fixed optimization design and Monte Carlo optimization design were compared. It is shown that the minimum transmitted angle derived from the Monte Carlo optimization design method is larger and the system has well transmitted performance and steering performance while the difference of steering error is small, furthermore movement precision is assured when design variables are little varied.