In situ observations of the matter in the earth interior under ultra high pressure and high temperature conditions by using hydrothermal diamond anvil cell connected to spectrometer and microscope, combined with synchrotron radiation technique, obtained new data and message of their molecular structures and natures in molecule and atom scale. The nature of earth fluids would be changed by the variations of the temperatures and pressures of their locations. It is well known that the properties of water in the critical region may show anomalies in many of its physical properties. For example, the density or dielectric constant of water may drop with increasing temperature. The molecule structures of aqueous solutions(NaCl H2O and NaCl D2O H2O) at high temperatures and pressures（800℃/3GPa and 25℃/10GPa) have been examined using diamond anvil cell in situ observation connected to FT IR spectrometer combined synchrotron radiation technique. The sharp and strong O H stretching vibration near the critical point of water proves the breakdown of the hydrogen bonding net work. Within and near the critical region, the rapid decrease in the dielectric constant of water is caused by the destruction of the hydrogen bond net work of the water molecules. O H vibration frequency increases with temperature，and also with increasing salinity. Experiments demonstrated that nature and structure of water (NaCl H2O) will change with temperature and pressure in the deep earth. Non hydrogen bonding of water would be predominated at high temperatures, which cause to form water clusters occurred at deep earth. As passing through the critical state，the anomaly properties of fluids will be caused by the variations of water molecule vibrations and hydrogen bond network broken at the critical state. In situ observations of the fluids in the earth interior under ultra high pressure and high temperature conditions by using hydrothermal diamond anvil cell will help us to understand the nature of the fluids，fuild rock interactions，and deep processes in lithosphere and in mantle in molecular and atomic scale.