Abstract: The position, size, development characteristics and their influence factors of bed separations have been analyzed deeply by experimental and theoretical methods. The results show that bed separations mainly occur below the key strata, and the highest bed separation space is right below the primary key stratum. When a combined breakage occurs on two adjacent key strata, even if the upper stratum is thicker and stronger than the lower one, there will be no bed separations below the upper key stratum. The largest width and size of bed separations after the key stratum breaks for the 1st time only reach to about 25%～33% of those occurred before the breakage. Therefore, grouting is best done before the key stratum breaks for the 1st time, and ensure that the key stratum would never break. In order to deal with the problem that the current bed separation grouting technology can not prevent the key stratum from breakage, the authors proposed the isolated section-grouting method for overburden bed separations by composing bed separation space grouting and pillar. This would be a technical progress in the bed separation grouting.