In this letter, we address the problem of how to measure the link quality in terms of signal-to-interference ratio (SIR) in rapidly time varying channels which are due to fast variation of the interference or the bursty nature of the traffic. We propose a new SIR estimation approach for the TD-CDMA system, that is based on a pre-selection tentative decision device which discards and selects the estimated symbol on the basis of MAP ratio. The simulation results show that the proposed SIR estimator works and is superior to other known estimators in rapidly time varying channels.