The studies on the domain evolution of MFIS or MFS structure in the retention process can be very helpful to investigate the mechanism of the retention loss in FeFET.Piezoresponse force microscopy (PFM) is a very powerful tool for observing ferroelectric domain and its real-time evolution.In this paper,we investigate the retention loss of MFIS structure with SrBi2Ta2O9 (SBT) as ferroelectric thin film and HfO2 as insulating buffer layer by PFM.