Laser-induced damage test of the sample is performed in a femtosecond laser system with duration 80 fs, centre wavelength of 2 μm. A wavelength separation multilayer of Ta2O5/SiO2（HT at 1.064 μm & HR at 2.128 μm）serves as the target, which is prepared by ion beam sputtering (IBS) on infrared glass. Meanwhile, damage morphologies of the sample are observed by Leica optical microscopy and scanning electron microscope (SEM). The laser-induced damage thresholds (LIDT) of the sample is calculated through the relation between damage area and laser fluence. It is found that damage morphology of the sample is layered, clear edges are easily seen around the damage spot, and the damage crater has no obvious sign of heat diffusion and conductance which means the femtosecond LIDT is closely related to intrinsic characteristics of materials. A theoretical model based on conduction band electrons is applied to discuss the phenomenon. Combining with electric field distribution and band-gap of materials we consider that the damage of the sample under 2 μm femtosecond laser pulse irradiation will first occur in the narrow band-gap material at the interface between high-index and low-index layers.