The influence of an applied water film on bone hard tissue ablation by pulsed CO2 laser is evaluated. Fresh bovine shank bone in vitro used in the experiment is put on a PC-controlled motorized linear drive stage and is moved repeatedly with the speed of 12 mm/s through focused beam of laser. In each sample, two cuts are produced, one with water film and the other with not. The wavelength of pulsed CO2 laser is 10.64 μm, the pulse repetition rate is 60 Hz, the energy density is 18～84 J/cm2, the beam diameter is about 400 μm and the water film tickness is 0.4 mm. The scanning number is five. The surface morphology and microstructure of ablation grooves are examined by steroscopic microscope and scanning electron microscope (SEM) respectively. The crater depth is measured with optical coherence tomography (OCT). It shows that water film can not only reduce carbonization and thermal damage, clean the ablation groove, but also augment ablation rate at the radiant exposure of 50 and 70 J/cm2.