In order to change the lane of a weeding robot to the navigation line, a visual navigation system and two reference frames were established, some relative navigation and measure algorithms were put forward. According to the condition whether robot’s moving direction were parallel with the navigation line or not, two different paths for lane changing were put forward and proved to be the optimal lane changing paths after the minimal turning radius was restricted. The corresponding control process and some calculation formulae about pivotal points and indispensable data related to the optimal paths were given. The two optimal paths planning methods for lane changing can be applied to any vehicle and robot with front wheel steering.