At present, the synthesis results of planetary gear train are saved in the form of adjacency matrices. Research on the sketching method of gear train mechanism is lacking. Adjacency matrices are not conducive to the kinematic analysis and evaluation of gear train mechanism. This study presents a new method for automatic sketching of planetary gear trains by generating a gear distribution matrix. The sketching rules of gears and planet carriers are established based on the structure characteristics. The gear is reasonably distributed by analyzing the gear meshing path and gear level. Thereafter, an algorithm of sketching for the planet carrier is proposed to avoid the line crossing between the planet carrier and gears. This study is beneficial in improving the design efficiency of planetary gear trains.