The typical neutral density filter is a metal film plated on a K9 glass to achieve the effective absorption of laser. Its lower damage threshold severely restricts its application to high energy laser systems. Experimental study on damage morphology and damage mechanism of filter in a higher laser energy density is carried out. The variation characteristics of damage morphology are as follows: with the increase of laser energy density, damage spots first appear on the filter, then develop into cracks, and the cracks grow gradually longer and eventually connect into linear and block forms, resulting in a large area of film dropping off. A model of defect absorption leading to film damage on neutral density filter is established. And temperature and stress distributions on the film surface are calculated, separately. The inhomogeneous temperature rise on film surface leads to radial, hoop and axial thermal stress distributions. Theoretical analysis shows that cracks along the radial direction are caused by hoop stress. When laser energy density is larger than about 2.2 J/cm2, impurity particle radius is larger than 140 nm and the distance between impurity particles is less than 10 m. A large number of cracks can connect together to cause a large area of film to drop off.