Abstract: Relationship between the link length and the angle of two neighbouring bodyworks is analyzed when a tractor-trailer mobile robot is moving.Then we optimize the robot structure by setting the link length.So,we minimize the minimal-turning-radius of the robot system under the precondition that the angle constraint is met.Then the tracking ability of the robot system is enhanced effectively.The conclusion can be used to guide the robot design.Furthermore,the idea of dynamic retractility control of the link is proposed.