The most critical reason for limiting the extensive study and promotion of MA-based perovskites is their intrinsic instability when compared to FA-based perovskites. Therefore, it is necessary to develop a simple and effective method to improve their intrinsic stability. Herein, the 1,1,1-trimethylhydrazinium cation (TMH+) was first introduced into MAPbI(3) to fabricate high-performance mixed-cation perovskite solar cells (PSCs) with an enhanced power conversion efficiency (PCE) of 19.86%, which benefits by the improved crystallization and morphology of films. On the one hand, the slightly large size of TMH+ is complementary to the low tolerance factor of MAPbI(3) and then enhances the structure stability. On the other hand, the presence of methyl groups in TMH+ is beneficial to promote the hydrophobicity of MA-based perovskite. More importantly, the hydrazinium group can effectively inhibit the production of Pb-0 in perovskites, which is the initial stage of degradation. As a result, the intrinsic stability of PSCs has been observably boosted. After aging at 45 +/- 5% RH for 1800 h and 85 degrees C for 200 h, the unencapsulated PSCs retained 77 and 79% of initial PCE, respectively. This work provides a new design for the selection of suitable cations with special structures and chemical groups to enhance the moisture resistance and intrinsic stability of MA-based perovskite at the source of degradation.