Abstract: The coarse grain HAZ (CGHAZ) is regarded as an area with the most weak mechanical property of the welded joint. The influence by the high temperature residence time (T_H) on the CGHAZ microstructure and the mechanical properties is studied by means of the test and the digital simulation, etc. As a result, the facts as following is revealed. Along with the increase of the T_H, the CGHAZ toughness is reduced, the diffusion coefficients of carbon and nitrogen atoms are increased austenite grain, and the M-A component distribution is homogenized;once the T_H is about 12 s, the CGHAZ presents best comprehensive performance;thanks to the digital simulation method, the process parameters are found out suitable for T_H as 12 s. The said method is used for the actual linepipe steel welding test, resulting in the overall mechanical properties of the welded joint up to the requirements as specified under the Technical Specification of the China-Russia Natural Gas Pipeline Project. Moreover the accuracies of the thermal simulation test and the digital simulation are verified, providing reference for optimization of actual welding process.