Due to errors in body manufacturing and assembly, it will create an original clearance between two plates. In order to study the influence of the original clearance on FDS, 6K21 aluminum alloy plate with thickness 2.0 mm and 6082-T6 aluminum alloy plate with thickness 3.0 mm were adopted as the objects to research relationships between the process parameters and the clearance between plates and how to reduce the influence by adjusting FDS parameters, which was verified by the joint mechanical properties. Taking the gap between plates and the maximum test force as the research index, using the control variable method and origin fitting,the following conclusions are drawn: the axial down force and tightening torque are linearly and negatively related to the gap between plates within a certain range; the original clearance will reduce the joint connection quality, the impact of the original clearance can be reduced by increasing the axial down force of 10%~20% or the tightening torque of about 20%,then the connection quality can be improved.