Bent inlet pipes are often used in centrifugal compressors due to limited installation space, and an understanding of the effect on compressor stability is essential for safety and dura-bility. This paper firstly investigates flow instability behaviors in two compressors, one with a straight inlet pipe and the other with an S-shaped bent pipe. In detail, it analyzes the resulting flow fields, instability evolution paths and surge boundaries. The results show that the S-shaped pipe obviously affects the flow field at high mass flow rates, while reverse flow mainly influences the flow field at low mass flow rates. Reverse flow first occurs at certain flow passages with a high pressure difference that is predominantly decided by the volute rather than the S-shaped bent pipe. In addi-tion, centrifugal compressors can tolerate reverse flow to some extent so that surge would not occur immediately if reverse flow occurs unless the reverse flow region extends circumferentially and radi-ally to a sufficiently large size. Since the S-shaped pipe is not dominant in the creation and extension of reverse flow, it does not exacerbate the stability of the central compressor to a great extent. Last but not least, the S-shaped pipe is noted to delay the occurrence of surge at 90％ rotating speed, which suggests the possibility of improving compressor stability with bent inlet pipes. This result differs from the conventional understanding that inlet distortion usually deteriorates compressor stability and emphasizes the particularity of centrifugal compressors.