We recently applied a geosynthetic-reinforced soil (GRS) technology to increase the resistance of tsunami barrier to an overflow erosion. In the present study, we performed a series of model tests in the laboratory to evaluate the resistance of previous and new tsunami barriers on the strength against the over-flowing tsunami. We examined dynamical balance of concrete panels covering on the crest and its outflow conditions. The test results indicated that the GRS seawall has a function of tsunami barrier that has a high resistance to erosion due to tsunami overflow. The length of geogrid layers required to prevent outflow of concrete panels was evaluated quantitatively under various tsunami heights.