Regardless of the impressive progress that perovskite solar cells (PSCs) have achieved, especially considering their power conversion efficiency (PCE) over 25%, traditional PSCs still contend with an inherent instability with exposure to humidity, which remains as a critical issue for the realization of commercial production. Herein, we proposed an effective pathway to relieve the instability of PSCs without sacrificing efficiency by introducing a 2D phase at the surface of the 3D perovskite film, based on a novel organic cyclohexylmethylammonium iodide (CMAI). The self-assembled thin 2D capping layer atop the 3D perovskite layer can not only reduce the ionic defects, but also serve as a protective barrier against moisture. Consequently, the champion device incorporating 2D perovskite capping layers delivered an open-circuit voltage (V_oc) of 1.19 V, which contributes to an impressive PCE of 22.06% on account of the improved charge extraction and decreased non-radiative recombination. More importantly, an excellent long-term stability along with mitigated hysteresis was observed for the modified devices as a result of the suppressed ion migration and high humidity resistance of the 2D perovskite film. Our finding provides a comprehensive approach for simultaneously enhancing the efficiency and stability of PSCs through dimension engineering utilizing CMA-based 2D perovskite materials.