Abstract: Based on Hill' elastic-plastic instability theory,the velocity field for sheet instability which is caused by the shear stress during sheet metal forming is developed,and the theoretical formula of the wrinkling critical shear stress is proposed.The influences on the critical shear stress of the yield strength,the strain-hardening ex-ponent,the plastic anisotropy,and the sheet thickness are analyzed,and are compared to the results of experiment and numerical simulation.The obtained results show that the wrinkling of sheet metal is induced by the effect of shearstress.The theoretical results for the overall tendency of the critical shear stress are similar to the experimen-tal and numerical ones,and the critical shear stress increases with the decreasing in yield strength σ_s,the in-creasing in strain-hardening exponent n,the increasing in plastic anisotropy r,and the increasing in sheet thickness t.