Since the earlier years of turbulence investigation, the origin of turbulence in boundary layer has remained a key challenge: whether it originates from the wall or comes from the external freestream turbulence.Progress in recent years has shown that there is a self-sustenance mechanism of wall turbulence which is almost not affected by the outer flow.This mechanism is closely related to generation of the coherent structure found in developed turbulence.However, it is also found that a "receptivity" mechanism exists for boundary layer flows, which characterizes the effect of freestream turbulence.So, what is the relation of these two phenomena? In this study, the idea of "energy gradient" is used to analyze the turbulent generation in the transition in boundary layer flows.