Radiation tolerant Micro Electro Mechanical System(MEMS) accelerometers are widely used in aerospace vehicles and nuclear industry. The reported researches are mostly concentrated on radiation hardness of MEMS devices' detecting circuits, while the radiation degradation mechanism of MEMS sensing part is still not well understood. Since MEMS device's micro-sensing part and detecting circuit must operate together, it is challenging to accurately distinguish the attributed degradation ratio of micro-sensing part when operating in the whole device, and acquire its radiation degradation mechanism by using micro and nano measuring techniques. To settle this issue, a radiation degradation measuring technique regarding to MEMS discrete parts is developed, and the radiation degradation attributed to MEMS sensing part is successfully extracted. Then, specifically fabricated MEMS contrast chips and electrical measuring techniques are utilized to study its radiation degradation mechanism. It is found that the radiation degradation attributed to MEMS sensing part is resulting from charging effect of the interface thin oxide layer, rather than charging effect of the parasite capacitor or changes of different resistor parameters. These trapped charges in the interface layer will produce an external electrostatic force on the movable proof mass, and then induce the output drift of MEMS accelerometer. In conclusion, this research experimentally infers a clear understanding of the degradation mechanism of MEMS sensing part under ionizing radiation environment, and it shows useful guidance on the future radiation hardness design of MEMS devices.