The core-crash sawtooth caused by m/n = 2/1 double tearing mode (DTM) in Tokamak is investigated using a three-dimensional toroidal MHD code (CLT). It is found that mode–mode coupling effect plays a crucial role during the core-crash sawtooth. The n = 0 component and n > 1 components generated by mode–mode coupling of the n = 1 modes take up more than 40% of the total kinetic energy and become unneglectable during the core-crash sawtooth. The n = 0 component is crucial for the saturation of DTM. Without the n = 0 component, the DTM will not saturate. The high n components are responsible for the explosive growth of the DTM that finally results in the fast crash of the core plasma temperature.