With the increasing applications of plastic gears, the low-precision manufacturing defects in injection molding of plastic gear appears increasingly due to the plastic shrinkage. On the basis of collecting domestic and abroad information and analyzing the problems of plastic gear design and manufacturing in applications, the relationship between plastic gear modification and rack tool modification is researched and ratiocinated, and the maximum tooth root fillet modification theory is proposed, and the question whether rack tool modification effects plastic gear modification during the manufacturing processing is discussed too. Finally, the trial-error theory is proposed in order to solve minimum tip thickness problem for small gear, and is applied on the design analysis of helical gear.