In this study,we apply generalized extreme value theory to the estimation of upper bound earthquake magnitude and the return level of strong earthquake for the potential seismic tsunami source and choose the Ryukyu trench subduction zone as the case study area. Firstly,we analyze the characteristic of geological structure and historical earthquake data,and delineate the potential earthquake tsunami source region. Then we divide the time domain into intervals according to the seismicity,and extract the upper limit earthquake magnitude samples of each time interval. Finally we establish the generalized extreme value model to estimate the upper bound earthquake magnitude and the return level of strong earthquake,and as well as analyze uncertainty of the estimation results.