To date, perovskite solar cells (PSCs) have achieved superior photovoltaic performance with a high power conversion efficiency (PCE) of over 22%. However, there are very few devices which have a high PCE and high stability simultaneously. In this study, we fabricated PSCs made from (FAPbI(3))(0.85)(MAPbBr(3))(0.15) using the non-volatile Lewis base acetamide (CH3CONH2) as an additive. Compared to a reference device, the device containing 5mg mL(-1) CH3CONH2 displayed a markedly improved PCE of 19.01% and less hysteresis, due to its high-quality film with a better crystal structure, evidently larger grain size and greater thickness. In addition, the device with CH3CONH2 exhibited better humidity and heat stability. The unsealed device could maintain about 70% and 50% of its starting PCE under around 50% and 80% relative humidity (RH) for 1000 h and 700 h, respectively. Meanwhile, the unsealed device with CH3CONH2 could retain about 80% and 60% of its starting PCE at 60 degrees C and 85 degrees C for 200 h and 150 h, respectively. These results clearly show that using CH3CONH2 as an additive can promote crystal growth and enhance the grain size of perovskite thin films and introducing a suitable additive can realize the simultaneous improvement of the PCE and stability of PSCs.