High-entropy oxides (HEOs) with an ultrathin geometric structure are especially expected to exhibit extraordinary performance in different fields. The phase structure is deemed as a key factor in determining the properties of HEOs, rendering their phase control synthesis tempting. However, the disparity in intrinsic phase structures and physicochemical properties of multiple components makes it challenging to form single-phase HEOs with the target phase. Herein, we proposed a self-lattice framework-guided strategy to realize the synthesis of ultrathin HEOs with desired phase structures, including rock-salt, spinel, perovskite, and fluorite phases. The participation of the Ga assistor was conducive to the formation of the high-entropy mixing state by decreasing the formation energy. The as-prepared ultrathin spinel HEOs were demonstrated to be an excellent catalyst with high activity and stability for the oxygen evolution reaction in water electrolysis. Our work injects new vitality into the synthesis of HEOs for advanced applications and undoubtedly expedites their phase engineering.