Abstract: In order to study mechanical property and failure characteristics of ultra thick top coal under front abutment pressure on fully-mechanized caving mining of ultra thick seam(over 20 m).we reduce confining pressure and increase axial pressure on the MTS815 rock mechanics test machine to simulate stress change of top coal.Accordingly,we carry out experimental study of coal sample size effect on different height-diameter ratio.Research result shows that coal sample exist test four stages of pore compaction - linear elastic - plasticity - destruction on different height-diameter ratio in triaxial compression.With the height-diameter ratio increasing,the performance characteristics of each stage as follows: the deformation of specimen increases at pore compaction stage; the elastic modulus of the specimen decreases at linear elastic stage; the axial strain of the specimen increases at plasticity stage; the post-peak stress drop becomes slower and the residual load strength increases gradually at destruction stage.With the height-diameter ratio increasing,the final failure mode of the specimen transit from fracturing destruction to shear failure.Fractures at the lower part of shear fracture surface develop well.On the contrary,there aren’t fractures at the higher part of shear fracture surface develop well.Thus,when we exploit ultra thick seam by the way of fully-mechanized caving mining,caving characteristics of mid and lower top coal are superior and higher part are inferior.