The carbon- free car is a kind of car with direction control function,the gravitational potential energy drive. In order to solve the manufacturing error,assembly error affect carbon- free car trajectory problem of inaccurate,through the car trajectory mathematical analysis,the trajectory equation is established. By using Mathematica software,the car trajectory simulation experiments is carried out. Then the sources of error and error forms affect the trajectory changes are analyzed and summarized. Based on this trajectory analysis and the design method of introducing the micrometer and worm gear mechanism into the fine- tune mechanism is proposed,a fine- tuning mechanism and fine- tuning method with higher adjust precision than ever are obtained. Through physical prototype testing,good results is achieved by fine- tuning program in the trajectory modification,a new way of thinking for carbon- free car fine- tuning mechanism design is provided.