Abstract:The effect of rock pressure on the crack of top coal of steep seam is analyzed by elastic2plastic theory. . It is put forward that the top coal of low gaseous steep seam can be caved only when the top coal is in the limiting equilibrium state of st ress under the abutment pressure. The sublevel height of top coal is calculated. However , the change of seam thickness has to be thought over when the sublevel height of top coal is determined. Therefore the seam thickness is considered as a random variable , and the reasonable sublevel height of top coal is determined by the reliability.