Direct numerical simulation (DNS) was carried out for laminar-turbulent transition of an incompressible boundary layer on a flat plate based on disturbance N-S equation in spatial mode with Fourier pseudo-spectral and massage passing interface(MPI) methods.The statistical data were analyzed.Transition processes caused by disturbances with different initial amplitudes were compared.Results indicate: after the amplitude of disturbances in laminar flow grows

nonlinear effect modifies the profile of mean flow

with enlargement of performed by the enlarging unsteady zones.At the same time

more and more high frequency harmonic waves are generated.When the profiles of mean flow are modified to certain degree

the unstable zones are enlarged significantly.More harmonic waves are actived and grown rapidly

triggering the breakdown of transition process.During the breakdown of transition process

the rapid growth of many harmonic waves induces the rapid growth of disturbance energy

accelerating the modification of mean flow profiles.The phenomenon means that the instability characteristic of mean flow profile is changed.The reciprocal rapid growth makes laminar flow become turbulent flow rapidly.Therefore

change of instability characteristic of mean flow profile plays a key role during process of transition.