In this paper, we propose a mechanism of solar flare based on the 3-D nonlinear mode coupling of resistive tearing instability. The results show that the nonlinear coupling of tearing modes leads the rapid destabilization of some high modes. Furthermore, tearing mode turbulence is formed and anomalous resistivity is produced, which in turn, quickens the development of tearing instability and accelerate the magnetic reconnection process. It is suggested that the fast magnetic reconnection as a mechanism of solar flare may be associated with this self-excited process caused by the nonlinear mode coupling of tearing instability in the solar corona. Using our model, we successfully explain all the main typical characters of the flares of February 4 and 6, 1986.