Abstract: The state transformation mechanism of top coal in top coal caving mining was studied with acoustic characteristics test in laboratory and experiment in situ. Also,the relationship between acoustic characteristics of coal sample and stress change under uniaxial compression was investigated. The results show that the ultrasonic velocity is stable when the load is applied gradually at the initial stage,it reduces sharply when the applied load reaches a certain value. The acoustic velocity is stable when the coal face is apart from the survey station (L≥12 m) in situ testing,but it reduces sharply (L≤12 m) with the coal face extraction. Concept of fracture degree is proposed based on the acoustic velocity of top coal in different media,and a degradation model can be divided five stages to reflect the top coal medium state transformation. The results provide the basis for identification of top coal,designing top-coal caving,mine pressure control and supporting structure.