Kinematics design is one difficulty point for complex planar gear trains. It will effectively promote innovating design and application of gear trains for computer to realize automatic kinematics design of the gear trains. Based on the demand of gear trains in mesh properly, the restriction of kinematics design of gear trains is analyzed, the relationship of the basic restriction with the structure of gear trains is presented. Thus, the information sets of the restriction equation for the kinematics design of gear trains will be gained. The kinematics design model of planar gear trains is constructed and solved by computer automatically.