The synergistic mechanism of Ni catalyst and supercritical water (SCW) during the treatment of dibutyl phthalate (DBP), a representative component of refractory phthalic acid esters, has been investigated using the ReaxFF molecular dynamic simulations. During the catalytic SCW gasification process, Ni catalyst could facilitate the bond cleavage while the roles of SCW were to promote cracking and serve as H sources. As for the evolution of dissociated side chains, the cooperative effects greatly enhanced the cracking of long-chain fragments and generation of saturated products. Ni could help split H2O and the SCW accelerated beta-scission as well as provided H and O atoms. For the aromatic ring-opening stage, a Ni-catalyzed process involved successive bond scission and migration of carbon chains into catalyst bulk was dominant while the SCW only offered marginal help. With regard to H-2 production, the synergistic effects were supposed to improve H-2 yield. Furthermore, the Ni catalyst was gradually deactivated during the catalytic SCW gasification process, which was caused by carbon deposition and surface oxidation. H-2 reduction method could remove the surface oxygen atoms from the Ni catalyst effectively to regenerate the deactivated catalyst to some extent.