By adopting a theoretical method of robust optimization design with design parameters and noise factors of normal distribution, a steering mechanism of truck is designed. A math model with clearances based on robust optimization is established. The movement precision in fourteen positions of steering mechanism during steering course is chosen as objective function, and kingpins distance, axle-base, the bottom angle of trapezoid mechanism and the length of steering arm are chosen as design variables, and clearances in locomotion joints are chosen as noise factors, and minimum transmission angle and maximum steering angle error are chosen as restraint conditions, etc. Optimum results showed that the kinematics precision of steering mechanism is assured even if there are clearances in locomotion joints.