AbstractMost western Northwest China (WNWC) has experienced notable wetting in recent decades. Despite the widespread attention of scientific community, the causes of such climate shift are still not well‐understood. In this study, we find that the dominant mode of Indian Ocean sea surface temperature variability — Indian Ocean Basin mode (IOBM) plays a crucial role in the precipitation over WNWC. Nonetheless, such a close lagged relationship is unstable, and varies with Pacific Decadal Oscillation (PDO). When the PDO is in its warm phase, there is no robust lagged relationship between April‐June WNWC precipitation variations and January‐March IOBM. When the PDO is in its cold phase, the warm phase of the IOBM inspires a wavetrain across the Eurasian continent in the next April‐June, with an uncommon low pressure around Lake Baikal, thereby enhancing the westerlies and thus more moisture into the WNWC region. This enables the IOBM‐based statistical rainfall prediction scheme to estimate the precipitation in the WNWC region with some confidence. This remarkable contrast in the influence of IOBM between the two phases of the PDO highlights exciting new avenues for improving climate predictions over the WNWC.