Oxygen evolution is the "bottleneck" step in the photocatalytic water splitting process, which can be settled by intentionally constructing a designed heterojunction and specially tuning the interface structure. Herein Bi6Fe2Ti3O18-BiOBr (BFTO/BOB) heterostructure with intimate 2D/2D interfaces are successfully grown by converting part of ferroelectric BFTO nanosheets to BOB via an ion-exchange chemical reaction. For the first time, it is discovered that the conversion of BFTO to BOB is a self-limiting process, i.e., the reaction auto-stops at a certain ratio of BFTO/BOB even high concentration of reactants (hydrobromic acid) are used. This phenomenon is ascribed to the precipitation-dissolution equilibrium due to the higher solubility product constant of BiOBr. The fine 2D/2D structure of ferroelectric BFTO/BOB with good energy band alignment leads to two times and seven times higher oxygen evolution rates than non-ferroelectric BFTO/BOB and pure BFTO under full-spectrum irradiation, respectively. The promoted separation and transfer of photogenerated electron-hole pairs at the interface is clarified by the soft X-ray absorption spectroscopies, Mott-Schottky curves and photocurrent tests. What's more, the ferroelectric spontaneous polarization at the interface between the ferroelectric BFTO and the BiOBr also shows a synergetic effect for the oxygen evolution enhancement. Our finds may open up new windows to prepare 2D/2D heterostructures with improved photocatalytic activities.