Diagnosis is undertaken of the genesis and maintenance of interannual anomaly in summer monsoon with the aid of the departure vorticity equation in the context of 1980～1986 ECMWF 200hPa gridded u-and v-winds.Evidence suggests that the features of the interannual anomaly flowfield over the Asian monsoon region exhibit appreciable intraseasonal oscillation; the features,e.g.,vigor and phase,as uncovered by the terms of the vortieity equation may differ more or less for different years and areas; striking difference exists in sign and magnitude of monthly means of the terms in this equation over China mainland for the drought and flood years; the climatology of monsoon circulation plays a critical role in the maintenance of interannual anomaly disturbance.Great difference in terms of the equation used for the late spring-early summer Meiyu rainfall may serve as a precursor of anomalous rainfall forecasting.