The effects of excess Si and Mg2Si content and the heat treatment process on the microstructure and mechani-cal properties of the Al-Mg-Si alloy automotive body sheets were investigated.The results showthat,the tensile strength ofthe sheets can increase obviously about 30% before simulated paint baking,and the elongation increases approximately15% with increasing the excess Si content.After simulated paint baking,however,the tensile strength of the sheets in-creases less than 15% and the elongation does not change significantly.With the increase of Mg2Si content,the strength of the sheets before and after simulated paint baking increases,but the effect on the elongation of the sheets is trifling.When the Mg2Si content increases from 1.25% to 1.97%,the tensile strength of the sheets increases from 316MPa to383MPa in T4P condition after simulated paint baking.The effect of pre-aging on the yield strength and elongation ofsheets is unnoticeable before simulated paint baking,which means that it cann′t affect the deformation ability of sheets.After simulated paint baking,however,its effect is remarkable.The value of383 MPa of tensile strength of the sheets canbe obtained,which is close to that of the tensile strength in T6 condition.The elongation of the sheets increases simulta-neously,which is beneficial to the improvement of performance of the sheets.