By three-point bending experiments on XY-plane of thin-walled super alloy honeycomb sandwich with different types and dimensions of penetrable defects, their failure modes and influence of defects of different types and dimensions on their mechanical properties are researched by observing failure modes and performance curves of test samples. Researches show that when failure occurs on sandwich structures under three-point loading, vertical to XY-plane, buckling depression occurs on center part of the upper plate along with fracture failure occurring on center part of the lower plate. Similar to three-point experiment on conventional structures, failure always occurs in the longitudinal regions near middle loading area. When lateral walls of honeycomb or strengthen points of welding are set at center loading area, fracture failure occurs on the lower plate, or only buckling deformation occurs on honeycomb cores. And by comparison, FE model can be used to evaluate its mechanical performance of thin-walled super alloy honeycomb sandwich with penetrable defects under three-point bending load. It is the experimental basis for improving the structural reliability and damage tolerance of the structure.