The abnormal phenomenon occurring in sensor calibration is an obstacle to product development but a useful guideline to product improvement. The sensitivity jump of micro accelerometers in the calibrating process is recognized as an important abnormal behavior and investigated in this paper. The characteristics of jumping output in the centrifuge test are theoretically and experimentally analyzed and their underlying mechanism is found to be related to the varied stiffness of supporting beam induced by the convex defect on it. The convex defect is normally formed by the lithography deviation and/or etching error and can result in a jumping stiffness of folded microbeams and further influence the sensitivity when a part of the bending beams is stopped from moving by two surfaces contacting. The jumping level depends on the location of convex and has nothing to do with the contacting properties of beam and defects. Then the location of defect is predicted by theoretical model and simulation and verified by the observation of micro structures under microscopy. The results indicate that the tested micro accelerometer has its defect on the beam with a distance of about 290μm from the border of proof mass block.