There is a pronounced antisymmetry with respect to the equator for the middle stratospheric methane, and it has been attributed to the anomalous annual cycle of vertical motions. The NCAR interactive chemical, dynamical and radiative two-dimensional model (SOCRATES) has been used to simulate and explore the possible cause of the anomalous annual cycle of vertical motions. The simulation results indicate that the quasi-biennial shift to the equator of high concentration ozone center plays an important role in the interannual anomaly of annual cycle. Ashigh concentration of ozone leaves the equator for the summer Hemisphere, there more solar radiation is absorbed in the middle stratosphere, while in the winter Hemisphere the situation is the contrary. Thus enhanced ascending motion is promoted by greater short-wavelength radiation heating in the middle stratosphere over the summer Hemisphere and strengthens the descending motion over the winter Hemisphere as well, which results in a stronger annual cycle. On the contrary, when high concentration of ozone is located close to the equator, there is a relatively weaker annual cycle.