In order to measure the dynamics of the ablation cavity which is generated in front of the fiber tip during Ho:YAG laser ablation in water containing material, we studied a fiber-optic laser probe method. The He-Ne laser beam as a probe laser simultaneously delivered through a silica glass fiber with the Ho:YAG laser beam as an ablation laser. The backscattering light of the probe laser from the fiber tip surroundings was measured at the laser input end of the fiber. We used water and agar as the water containing material. The measured backscattering light might be mainly taken place by the debris scattering in the ablation cavity. The transient intensity change of the measured backscattering light was predicted to offer useful information about the growth and extinction of the ablation cavity. From the experiment, the e-holding decay time of the backscattering probe light waveform indicated good agreement with calculated cavity collapse time in water using the Rayleigh equation. We also found that this e- holding decay time consisted with the theoretical cavity collapse time, which could be obtained by the viscoelastic model of agar. Therefore, we concluded that the e-holding decay time of the backscattering light waveform revealed the cavity collapse time of the ablation cavity, which is influenced by the material viscosity. We measured the e-holding decay time of vascular samples. Each tissue had individual e-holding decay time which might be explainable by the tissue viscosity.