The development of sunlight as the driving force to purify the marine environment is an energy-saving, environmentally friendly and sustainable technology. The construction of photocatalysts with morphology normalization is expected to guide the adjustment of light absorption properties and electron transfer paths, thereby enhancing the photocatalytic performance. Herein, the peony flower-like Bi₂O₂CO₃ (BOC) with morphology normalization was successfully prepared by the simple hydrothermal technology in the presence of polyvinylpyrrolidone (PVP) as polymer microreactor. The normalized peony flower-like BOC effectively broadens the light absorption range, improves the carrier separation efficiency, and prompts more active oxygen species (•O₂⁻) generation. Compared with the amorphous BOC, the photocatalytic degradation rate of organic dyes for normalized peony flower-like BOC is increased by at least 2.3 times. The normalized peony flower-like BOC with good ion anti-interference ability exhibited excellent photocatalytic performance for decomposing organic pollutants in real seawater. The degradation efficiency of RhB in seawater under natural light is 98.7%. The detailed degradation intermediates were probed in depth by liquid chromatograph-mass spectrometer, and speculate the possible photocatalytic mechanism. This work opens up fresh ideas for the design of photocatalysts on efficient seawater purification.