The daily grid precipitation data of National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) is used to analyze the spatial-temporal change of extreme precipitation in the Tianshan Mountains from late spring to early summer (May and June) and the mechanism of Indian Ocean Basin Mode (IOBM) on extreme precipitation. The results show obvious spatial variations of extreme precipitation in the Tianshan Mountains from late spring to early summer. Extreme precipitation in the Western Tianshan Mountains significantly increased, while other regions hardly varied. The diagnosed and numerical simulation results consistently showed that the increase of extreme precipitation in the Western Tianshan Mountains related to the coetaneous positive anomaly of IOBM which promoted the convergence of warm and cold airflows in the Western Tianshan Mountains. On the one hand, the positive anomaly of IOBM strengthened the anomalous anticyclonic located in the eastern Europe to north central Asia which promoted the cold airflow southward transportation. On the other hand, it induced the non-uniform warming of Indian Ocean, which resulted abnormal vertical circulation and its subsidence caused the anticyclone anomalies in the Arabian Sea and the Indian peninsula. The anomalous anticyclone and southerly airflow jointly conveyed warm moisture from the Indian Ocean to the Western Tianshan Mountains, which was conducive to the increase of extreme precipitation in the Western Tianshan Mountains.