The observed tropospheric biennial oscillation (TBO) in the weste North Pacific (WNP) monsoon region has an interdecadal variability with a period of 40-50 yr.That suggests a weaker effect of the TBO on the East Asia followed by a stronger one.A simple analytic model was designed to investigate the mechanism of the interdecadal variability of the TBO.The results indicated that a local TBO air-sea system not only supports the TBO variability in the WNP monsoon region but also produces an interdecadal variability of the TBO.