By using the 3- Dimension tolerance analysis software CETOL 6σ,taking the gear backlash in the gear system as closed- loop,considering the actual assembly sequence and the standard normal distribution of the processing size of each part,the 3D dimension chain analysis is carried out,the size of the backlash is obtained,and comparing with the result of existing calculation method. Combining the result of contribution degree analysis,optimizing the tolerance and the backlash is taken as the input substitutes into the established one- dimensional dynamics model of gear to analyzing its dynamic response.