Abstract: The production of oil and gas is a process that the oil and gas fluid flow under high pressure in porous media. The high pressure property of stratum fluid is a very important parameter. How to increase the testing accuracy of the stratum fluid, it is an important significance for the test of formation fluid high
pressure property. According to the testing theory of laser, it is considered that the change of the scatter section in gas and liquid phase led to the change of the intensity of light through the gas. We established a set of testing device of laser. The test equipment proceeded to test repeatedly at several temperatures,the results were compared with the Handbook of Physical Prop erties of Liquids and Gases, and it was proved that our results
were more accurate than that of the Handbook. It provided the foundation to further study the phase change in the porous medium.

Key words: laser testing, dew point pressure, phase state