Gap,misalignment and non-centering of probe (NCOP for abbreviation) are three important elements of the assembly quality of friction stir welding joints.Actually, these three elements have combining effects on the joint performance. In this paper, aiming at AA221 that is commonly used material in aerospace welding structures, the mathematic model between these three elements and the weld joint performances is established by the response surface methodology.The influence of each element on the weld joint is analyzed.Through experiments, the gap is found to be the key element on the weld joint performance, and the amount of misalignment and NCOP will cause the weld joint performance to show asymmetry.This method provides a mathematical model for the quantitative description of weld assembly quality.And, the mathematic model is used to provide reasonable instruction for these three elements of assemble quality of weld joints. This method avoids the disadvantages of traditional way on individual assembly elements and has good engineering application prospects.