Abstract: According to different damage and deformation characteristics of soft and hard rock, and with analyzing the rock's limit load and limit strain when rock breaking occurs, inhibition mechanism of soft rock on fracture development is founded. It is proposed that fracture development would stop when there are several thick soft rocks at a relatively high position or soft rocks at a relatively low position below the rock whose inferior free space's height is as low as 0, as the height of key layer is determined. Comparing with the thin soft rock, the thick soft rocks at a relatively low position have a bigger influence on the height of fissure zone. Taking N1102 and S1206 working faces in Tunliu Mine as examples, and analyzing lithologic characteristics of the overlying rocks, these are concluded that the height of goaf fissure zone in N1102 working face is from 23.6 m to 30.5 m, and the other is from 32 m to 34.5 m. The accuracy of this method was well validated with water leak method.