The influences of the focal length and the focal position on the damage probability and the damage morphology near the front and the rear surfaces of K9 glass were studied by using the millisecond laser damage test platform. Results show that, the melting process is dominant when the laser is focused at the front surface. While the stress induced damage is dominant when the focal plane is at the rear surface, and the damage size is obviously larger than that of the front surface. A two-dimensional thermal stress model was established to calculate the temperature field and the thermal stress field. Results show that the radial stress and the hoop stress are the main factors leading to the stress damage. The laser supported combustion wave induced by the laser irradiation at the front surface can enhance the energy coupling efficiency, which is one of the main reasons for the melting process at the front surface. Moreover, for the shorter focal length, the damage probability decreases rapidly with the increase of the distance between the focal plane and the sample surface. While for the longer focal length, the front and the rear surfaces can be damaged simultaneously when the laser is focused at the front surface, which can be attributed to the focal depth of lens.