The dynamics characteristic of a double-disc rotor with two breathing cracks is studied. The dynamic equations of the rotor system have been established based on FEM. The stiffness matrix of the crack element is calculated by strain energy release rate. A truly breathing behavior is represented by taking into account the opening and closing of the crack using stress intensity factor. The variation law of the stiffness in a steady-state rotating cycle is analyzed, and the effects of the crack depth and the crack angle to the vibration response are discussed.