Hydrogen gas is usually included in coal gas environment, so hydrogen induced permeation would happen to pipeline, especially in welding heat affected zone (HAZ). Hydrogen permeation process in pipeline is the preconditions for the following hydrogen embrittlement failure. With the development of coal gas industry, the basic research to the hydrogen permeation behavior in pipeline under coal gas circumstance is still unfortunately lack and urgently needed to supplement. In this work, X80 pipeline steel was used, and the HAZ samples, including intercritical heat affected zone (ICHAZ), fine grained heat affected zone (FGHAZ) and coarse grained heat affected zone (CGHAZ), were experimentally simulated using a Gleeble 3500 simulator. Next, hydrogen permeation tests were conducted on X80 pipeline steel and HAZs in coal gas environment. Calculated results indicated that the hydrogen diffusion coefficient increased with the rise of peak temperature in HAZs, but it was opposite to other parameters, such as sub-surface hydrogen concentration, hydrogen solubility and hydrogen trap density. The mechanism of the difference in HAZ hydrogen permeation parameters was analyzed combined with OM, EBSD and TEM analysis. It turned out that the content of large-angle grain boundaries, the grain boundary straightness and dislocation density were the main factors, where the large-angle grain boundaries and dislocations could dramatically arrest hydrogen atoms while the straight grain boundaries may act as hydrogen diffusion path. For FGHAZ, the straight grain boundary and low dislocation density compared with matrix played the predominant role in hydrogen diffusion process, and thus the hydrogen diffusion coefficient increased compared with steel substrate. For ICHAZ and CGHAZ, the decrease of large-angle grain boundaries and dislocation density acted as the main factor, especially for CGHAZ, the microstructures was mainly composed of tabular bainite ferrite (BF) with large grain size and straight grain boundaries because of the highest peak temperature, and the content of large-angle grain boundaries decreased obviously. In comparation with other regions, CGHAZ had the highest hydrogen diffusion coefficient and the lowest hydrogen trap density and hydrogen solubility.