An assembly tolerance optimization design method based on SDT tolerance modeling and Monte Carlo simulation was proposed.Through analyzing the relationship among the three core elements,namely the assembly accuracy chain,assembly joint surface and joint surface tolerance,the tolerance model was established using SDT theory and Monte Carlo simulation method,then homogeneous transformation theory was used to establish the joint surface error model and assembly accuracy model,and then the mapping between assembly accuracy and joint surface tolerance was deduced.Combining tolerance manufacturing cost,assembly accuracy reliability principles,a tolerance optimization model for assembly accuracy was established.Finally,tolerance optimization analysis was carried out with the top assembly as an example.It is found that,under the premise of meeting the reliability of assembly accuracy,the processing cost of the top assembly can be reduced by 9.02%,and the reliability of assembly accuracy can be increased to 97.81%,which verifies the effectiveness of the proposed method.