For CsPbBr3 perovskite materials, it is especially important to reduce interface defects, suppress non-radiative recombination, and improve morphology to achieve highly efficient and stable CsPbBr3 perovskite solar cells (PSCs). Herein, we reported a facile but highly efficient approach in additive engineering for improving the efficiency and stability of CsPbBr3 PSCs. It was found that phenethylammonium iodide can passivate interface defects, suppress non-radiative recombination, and increase the grain sizes of CsPbBr3 films by optimizing crystal quality and interface contact. As a result, a carbon-based CsPbBr3 PSC with power conversion efficiency > 8.51%, storage stability > 340 days, and excellent harsh stability under high temperature and humidity, has been achieved.