Abstract: The high sensitive laser interferometer of checking minuteness phase shift 0.2° was built using the way of heterodyne and phase track. The line mass-density of gas puff Z-pinch load was measured. The interruption of machine vibration to interference-signal was eliminated by putting the interferometer into the vacuum chamber and setting the laser and vacuum chamber and optic elements on the full gas optical platform. The curve of average line mass-density of Ar gas load varying with time was obtained. It can be used to optimize the Lavale nozzle design. The forming time of gas fluid stationary state is helpful for adjusting the synchronization between the opening time of electromagnetic valve and the drive current from pulse power facilities in Z-pinch experiment.