Abstract: The aspheric primary mirror of new generation space camera is made up of some ultrathin segments. Since the mirror is ultrathin and the material of the mirror is brittle, it will be broken easily if the force exerted is not proper. Therefore, the damage threshold of ultrathin mirror should be studied. The concept of stress intensity factor in elasticity was applied to active optics and stress intensity factor was proved to be the material′s damage threshold. It is put forward that microcrack in the material is the major reason for breaking. A method to calculate stress intensity factor is introduced. At the end, the model of an ultrathin flat mirror was actually calculated, the relation between the damage threshold and the size, thickness, mounting and roughness of ultrathin flat mirror was achieved.