Abstract：Based on the stability mechanism of the surrounding rock for large and small structure of the gobside entry retaining,it was analyzed that the structure of the key block up the gob-side entry retaining and its surrounding rock stress environment,as well as the key block length impact on the structure of surrounding rock and the filling body. It is thought that the key block length is the key factor maintaining the stability.Combined with the field geological conditions,the reasonable length of key block was obtained according to result the numerical simulation in gob-side entry retaining of Pingdingshan No. 12 mine with thick hard rock roof,and the length of key block was optimized by using the cutting roof technique of tendency small horizontal angle drilling group.

Keyword：thick hard rock roof;gob-side entry retaining;key block;reasonable length;