The invention provides an on-line sampling apparatus for the metal nanoparticle fluid of the vacuum submerged arc process and the method thereof, using the principle of the pressure difference between the vacuum pump and the vacuum chamber and the constant temperature design of the sample pipeline to make the sample precisely be caught and flow into the predetermined collector, and the disadvantage of the vaporization of the sample due to the temperature rise caused by the ambient temperature can be prevented. Further, the invention integrates with a particle size analysis apparatus to carry out real time measurement and data analysis of the nanoparticle fluid with real time process characteristics, wherein the nanoparticle fluid is caught from the nanofluid process line, thus, the optimal design work of the process and the system parameters and particle quality monitoring may be proceeded efficiently.