In this paper, the damage process and mechanism of diode pumped alkali vapor laser (DPAL) windows have been experimental studied and analyzed. The results show that two different processes exist in DPAL windows (take glass as example): the heat induced physical damage and the reaction (between alkali and hydrocarbon) induced chemical damage. The mechanism of these two processes has been analyzed. The research method in this paper can be applied to study the performance of other DPAL windows.