Abstract: In order to study the law of roof rock breaking and its main characteristics of the No.9 coal of Pingshuo mine in the effect of the No.4 dynamic pressure, with observation analysising the No.9 mining face working resistance of hydraulic support and the fracture development of floor in the effect of the No.4 dynamic pressu re, researched on the mining law of fully-mechanized caving face in ultra thick coal seam under goaf and analysised the roof damage of No.9 coal and effect on the roc k pressure.The study results showed that there were two mine stratas, grit and sandy mudstone included in mining face roof. The weighting step of the sandy mudstone was 9.8 ~ 24.0 m, with an average of 17.7 m.The weighting step of the grit is 12 ~ 34.4 m, with an average of 22.6 m.The dynamic load coefficient was 1.43 ~ 1.62, with an average of 1.56.The weighting step of the grit were larger and the weighting step were effected easily by fracture.The longitudinal fissure is the main destruction of t he roof of the ultra thick coal seam under the goaf.