Abstract: In this paper, different mechanical models for rocks are established according to the geological and hydrogeological conditions of the leading water fault, with careful consideration of the various factors which influence the size of the waterproof coal block nearby the fault. Applying finite element method on the theory of rock mechanics. the authors repeatedly calculate the stresses and the changes of the stresses after the coalbed mined on the conditions of different sizes of the coal block. And in addition, the authors analyse and study the stability of the coalbed and the rock formation in a state of changed stresses by No Tensile Stresses Analysis Method. On this basis, much more reasonable size (replaced by H_biock) of the coal block is determined H_block=F^e(σ^-)|σ^-=R_trm With further explanation of practical examples of a mine,the results are more idea.