In this paper, failure mechanism of DT 4H-SiC power MOSFET under unclamped inductive switch (UIS) test is evaluated by combination of experiment and theoretical research. The results show that unlike planar 4H-SiC power MOSFET, the gate oxide at the corner of of gate trench is destroyed under the UIS test, therefore, the device under test failed. And then, measurement results of the gate leakage and resistance between gate and source (R_gs) of the failed device indicate that gate leakage increases sharply and R_gs is only 25 Ω, however, the threshold voltage of failure device is unchanged. The analysis of the inner electrical field under avalanche state by using the TCAD software shows that the maximum electrical field exists at the corner of gate trench and the maximum junction temperature does not exceed the melt point of metal. These results are consistent with the experimental results .