Abstract: Temporal mode was used to simulate numerically the breakdown process of laminar-turbulent transition and the fully developed turbulence in an incompressible boundary layer on a flat plate.The analysis of results showed that,before the breakdown process of laminar-turbulent transition,the growth rate of amplitudes of a two-dimensional and two oblique Tollmien-Schlichting waves has already deviated from the linear growth rate due to non-linear interaction,and energy transfer existed between different wave modes.During the breakdown process,the rate of strain had great effect on the general stress,and the Reynolds stress has remarkable effect on the general stress after transition.The conclusion indicates that many more harmonics will be quickly excited,and their amplitudes will also increase quickly,the direct consequence is that the Reynolds stress and its corresponding normal-wise gradient increase greatly,which in turn further modifies the mean flow profile in a much faster pace,leading to the breakdown of the laminar flow with a catastrophic nature.