Abstract: Chain instability of residual coal pillars can lead to the dynamic disaster,such as the overburden caving,surface collapse,dynamic shock,gas leaking and water releasing.Revealing the chain failure mechanism of residual coal pillars is the basic prerequisite for its accurate control.Starting from the chain instability source,the weakest failure model of residual coal pillars was firstly proposed.The basic concept of key pillar was defined and its characteristics was analyzed.The determining method of key pillar were developed and the chain failure of residual coal pillars was revealed.Then,the key pillar theory in the instability of residual coal pillars was formed.At last,the potential application scope and field of key pillar theory was discussed.Results show that ① the differences in the internal physical-mechanical properties and external environmental factors have led to the weakest instability model of residual coal pillars.When any form of instability models appears in the residual coal pillar system,the weakest instability model must have occurred.That is,the coal pillar with the weakest stability factor fails inevitably when the chain instability of pillar system occurs.② The key pillar refers to the pillar that the local instability occurs earliest in the pillar system.Only the local failure of key pillar appears,the collapse of adjacent coal pillars with stronger stability can be activated.And the chain failure of residual pillar system can occur.③ The pillar with the smallest safety factor can be determined as the key pillar in the pillar system.Four principles of regionality,relativity,dynamics,and compound should be followed in determining the position of key pillar.④ Linear increase of load for the nearest adjacent coal pillar is triggered by the gradual reduction of load for key pillar.That is,the local instability of key pillar will cause the shift of overburden stress,which will transfer to the nearest coal pillars and lead to further instability and damage.Eventually,the chain failure and destruction of residual pillar system may also be caused.⑤ The key pillar theory can be used in the fields,such as the adjacent seams mining in the pillar areas,the strong pressure behavior controlling,the coal pillar designing,the backfilling mining,the gas extraction and the water disaster prevention.It can also be applied to the controlling of rock pillars in the non-coal resources mining.The key pillar theory in the instability of residual coal pillars is expected to promote the development of the theories and technologies of Chinese green mining.