Abstract: Abstract: The support of extra thick-compound roof is difficult and very easy to cause large area roof fall accidents. In the process of supporting ,damage and fracture of bolt and anchor cable often appear. And the process of roof fall is very fast and lack of warning, which is more difficult to prevent. In order to solve the problem, this paper ,firstly, sorts out and summarizes the definition and characteristics of extra thick-compound roof ,and, secondly, analyses the failure process and instability mechanism and reason of anchor cable easy damage and fracture and the roof caving process is fast. The results show that the reason of anchor cable is easy to damage is that the shear stress is bigger; the main reason why the thick-compound roof is more difficult to support is that: on the one hand, the thickness of strata is smallerd and the majority of the poor lithology, which will make rock easily destroyed one by one and more loose to support expansion force; on the other hand, for the thickness is large, the anchor is not available to reach to the deep stable rock and anchor in cranny rock will gradually failure as the expansion of fractured rock and rock pressure increases; the reason why thick compound roof falling is rapid and lack of warning is that there is one or several layers of hard rock in its internal and the hard rock gradually accumulated relatively more energy in the process of the energy savings and it is active against the surrounding rock deformation and failure. When the energy savings reach to a limit and lead to destroy and the destruction of soft and broken rock cannot bear hard rock which is transferred from the great energy, a sudden collapse occurs; numerical simulation results show that whether the thick-compound roof lithology is good or bad plays a key role in the supporting difficulty degree and grouting reinforcement less rock layer on a support will cause a multiplier effect. "