We investigate the damage characteristics and mechanism of neutral density filters consisting of metal film on K9 glass substrate by 1064-nm pulsed laser. The damage morphologies present marked differences with different laser pulse energies. Specifically, with the increase of laser fluence, the damage pits density increase as well, and at the same time, the cracks appear around the pits and interconnect, which lead to the abscission of film. The damage mechanism has been studied from the viewpoint of embedded impurities in the film. The theoretical results show that the difference between the thermodynamic properties of impurities and film can lead to thermos-elastic stress, which plays important roles in deformation of film, nucleation and propagation of cracks. Last, methods have been proposed to improve the laser damage resistance by controlling the size distribution of impurity particles and increasing the film tensile strength.