The microstructure especially the size, shape, number and distribution of precipitate, together with the strain hardening exponent n value at different strain range during plastic deformation of the Al-0.9Mg-1.0Si-0.7Cu-0.6Mn alloy sheet, subjected to different heat treatment were investigated. The results showed that the strain hardening exponent n values of Al-0.9Mg-1.0Si-0.7Cu-0.6Mn alloy sheet at any different strain range are different from each other, which is in agreement with the result that the relationship between true strain and true stress of polycrystalline alloy sheet during tensile test does not fully meet the Hollomon formula. The continuous strain hardening exponent nc defined in this paper essentially represents the approximate liner strain hardening effect during the total calculating strain range, while the stage strain hardening exponent ns defined in the paper can objectively indicate the counteraction of the micro strain hardening with the micro strain softening of alloy sheet during plastic deforming. When the precipitate in the matrix of alloy sheet can be cut by dislocation, the alloy sheet has the weakest strain hardening effect at the beginning of yielding process. Otherwise, the alloy sheet has the most prominent strain hardening effect at the beginning of yielding process when the precipitate in the matrix can be bowed bypass operation of dislocation. Gridded precipites is of no advantage to the glide and multiplication of dislocation of alloy sheet.