In order to compile and select specification of pressure piping design, construction, installation and operation correctly, and ensure the long-term safe operation of pressure piping, it is necessary to analyze the principle of pressure piping classification theoretically. This paper established the theoretic basis of pressure piping classification based on the two elements, functions and attributes of pressure piping, and explained the importance of pressure piping classification principle by showing the history of ASME B31.3 Process Piping. Combining with specific pipeline analysis, the classification principle is put forward and factors are listed as in following sequence:function, flow medium, terminal target/end users. Secondly, because hydrogen utilization is developing rapidly, this paper compared and analyzed ASME B31.12 Hydrogen Piping and Pipelines, ASME B31.3 Process Piping, and ASME B31.8 Gas Transmission and Distribution Piping Systems from the principal of pressure piping classification angle. To the end, this paper offered suggestions on China’s development of steel pipe which can inhibit hydrogen embrittlement, and specification on hydrogen pipeline(including in-service mixed natural gas/hydrogen pipelines).