The invention relates to a preparation method for increasing damage threshold of a film used by a Tm optical laser endoscope, belongs to the field of film optics, and is mainly targeted at the bottleneck of the film used by the Tm optical laser endoscope, of which the wave band is 2 micrometers, namely water absorption of the film. A laser, of which the wave band is 2 micrometers, is positioned at the strong absorbing region of water and water vapor, comprises water absorption peaks such as 1.94 micrometers, 2.09 micrometers and 2.94 micrometers; existing researches prove that the major factor causing a low damage threshold of the laser film, of which the wave band is 2 micrometers, is water absorption inside the file. The preparation method, provided by the invention, adopts the ion source assisted sedimentation technology with the ion source voltage range of 600-900 V, adopts two coating film source materials of metal Hf and quartz ring SiO2 with sedimentation speeds of 0.15 nm/s and 1 nm/s respectively, reduces water content of the film, and even eliminates water in the film; the preparation method for increasing damage threshold of the film used by the Tm optical laser endoscope is thus provided. The method can greatly and remarkably increase the damage threshold of the film used by the Tm optical laser endoscope, and has the characteristics of high pertinence, high efficiency, and simplicity and feasibility.