Monsoon boundary zone (MBZ) is a transitional zone between the arid Central Asia (ACA) and humid Asia monsoon area, located in North China-Mongolia. During boreal summer, both the mid-latitude westerlies and East Asian Summer monsoon (EASM) play essential roles in the precipitation variations in the MBZ, via causing anomalous cold air mass from the west and warm/humid air from the south to the MBZ. In this study, we defined a summer westerly index (SWI) over the key westerly domain (35 degrees-42.5 degrees N, 80 degrees-100 degrees E) and an EASM index (EASMI) over the key EASM domain (25 degrees-35 degrees N, 107.5 degrees-125 degrees E) to investigate westerlies-monsoon interaction and their effect on MBZ climate. The results show that westerlies and EASM have a synergistic effect on the precipitation in the MBZ and this synergistic effect could be amplified by the westerlies-monsoon interaction. The westerlies and EASM interaction could induce a local cyclonic anomaly in the MBZ, this cyclonic anomaly further intensifies the westerlies and monsoon flow through a dynamical amplification, favours the precipitation in the MBZ. The MBZ precipitation also contributes to maintain the cyclonic anomaly via the latent heating release. The interannual variability of westerlies is largely modulated by the mid-latitude Silk Road pattern (SRP) and the meridional displacement of the Asian westerly jet (AWJ), and the EASM variability is mainly modulated by El Nino-Southern Oscillation (ENSO), the combined effects from these mid-latitude circulation systems and tropical SST contribute to the climate variability in MBZ.