Abstract: The percolation phenomena, which are that with some kind of critical factor changing, the original properties of the object will sudden change, widely exist in nature. And when the changing is happening, the value of the critical factor is the percolation threshold. The percolation phenomena also exist in the process of rock damaging and failure and the percolation threshold can be studied by renormalization group approach. Assumed that the failure of red-layer soft rock cased by the failure of cement in structure. And according to the microscopic structure of silty mudstone, one kind of red-layer soft rock, can build the renormalization group model of red-layer soft rock. In order to facilitate the calculation, the rules of renormalization transformation set simply. Then the percolation threshold of red-layer soft rock failure under loading condition can be calculated by the renormalization group approach. When the damage extent of red-layer soft rock failure under loading condition reached the percolation threshold, the inner structure will damage in large-scale and the strength will drop rapidly. The random damage of red-layer soft rock change into ordered damage, the velocity of damage becomes faster.