Abstract: Based on a general discussion about the nonlinear instability characters of the basic state or the seasonal cycle of the climate system, this paper studies the nonlinear interaction between interannual variability (or rather the ENSO) and the seasonal cycle. Oxford model, a coupled air-sea model, is employed to demonstrate this theoretical analysis. Theoret ical and numerical studies indicate that an ENSO-like irregular oscillation of coupled air-sea system can be produced after a series of bifurcat ions which are associated with the loss of stability of annual cycle, the appearance of a new period, its phase locking with annual cycle and the chaos. This provides the basis for both the understanding of the mechanism of ENSO and the parameter determination of a coupled air-sea model.