Abstract: The contribution of the IB correction to the annual constituent S_(a) and other principal tides is evaluated.When the IB correction is applied with the MGDR or calculated with an average pressure for each cycle,the discrepancy both are large,and the difference between them is small. But if IB correction is not added to the altimeter ranges,the discrepancy is almost reduced.Through comparing the difference of harmonic constant of S_(a),the average contribution of seasonal air pressure variation in amplitude of S_(a) is(about) 42%,and the seasonal air pressure variation causes the phase lag of S_(a) decreasing 27°.So IB correction should not be added to the altimeter range,and the tide model of S_(a) is essential to the annual variation to be removed.Therefore an S_(a) model covering China sea is established.