Based on our newly constructed W–Cu potential, pure W and W–Cu alloys are simulated with compression and shear deformation to reveal the fundamental mechanisms of the BCC-FCC phase transition. It is found that the BCC-FCC phase transition of W have two different mechanisms, i.e., BCC W transforms into FCC W by the Bain and Nishiyama-Wassermann (NW) paths under compression and shear deformations, respectively. Simulations also reveal that the addition of Cu promotes the BCC-FCC phase transition of W through the NW path, while it prevents the BCC-FCC phase transition through the Bain path. Moreover, the slips of 16aBCC[011]BCC(011‾)BCC and 16aBCC[01‾1‾]BCC(011‾)BCC on both sides initially bring about the nucleation and growth of FCC W. The corresponding topological models, volumetric strain, as well as the changes of lengths and total energies are also discussed to provide a fundamental understanding of the intrinsic mechanism of the BCC-FCC phase transition.