The hole-transporting layer is an essential component in a perovskite solar cell (PSC) and plays a key role in controlling both power conversion efficiency (PCE) and stability. Here, we report a new hole-transporting material (HTM), methoxy group-containing poly(triarylamine) (PTAA) (CH3O-PTAA), for efficient PSCs with improved thermal stability. As compared to commonly used PTAA (CH3-PTAA), CH3O-PTAA exhibits enhanced doping ability and stability under thermal stress. With CH3O-PTAA, (FAPbI(3))(0.85)(MAPbBr(3))(0.15)-based PSCs show high PCEs over 20%, comparable to those of CH3-PTAA devices. More importantly, better long-term thermal stability with only 3% reduction from the initial PCE (6.1% reduction on average) has been achieved for encapsulated PSCs with CH3O-PTAA than that of PSCs with CH3-PTAA under dark storage conditions (ISOS-D-3) of 85 degrees C and 85% relative humidity (RH) over 1000 h. Detailed studies have been conducted to reveal the strong correlation between the doping behavior of HTMs and the performance of PSCs, which provide useful guidelines for the design of new HTMs for efficient and stable PSCs.