Abstract: Firstly, a geometric feature error model is established based on small displacement torsor. The actual variation bandwidth of error parameters and their functional relationship with tolerances are obtained through Monte Carlo simulation and response surface method. An error model of mating surfaces is established to study the error transmission properties in both series and parallel mating. Secondly, a tolerance optimization model is proposed, targeting tolerance cost and constrained by assembly precision reliability and tolerance principles. Finally, the tolerance optimization analysis for the spindle cutter head component of a bevel gear milling machine is completed. It is found that under the premise that the reliability of assembly accuracy is not less than 97%, the cost of tolerance machining is reduced by 8.36%, which indicates the value of this method in guiding tolerance design.