Abstract: Mechanical damage are easy to generate during the whole process of harvesting, classification, packaging, transportation, processing, preservation and selling, etc. Damage in threshing while harvesting is one of the most important source. Researches of grain damage during threshing at home and abroad were introduced. Methods about how to detect damage are considered, which include direct observation, colorimetric determination, laser optical reflection, image processing, pattern recognition, machine vision and MRI, and damage quantization are also explored. Finally, analysis are made on grain damage during threshing in China.