This paper simulation investigates field temperature of the Auto-flux core arc welding process (Auto-FCAW) in the one side multi-pass butt joint in position vertical. The material used steel group A, mark steel NVA36 by the Det Norske Veritas, yield strength: 355N/mm2, thickness 13mm. In this research, simulation thermo-elastic-plastic finite element (FE) menthod by using SYSWELD 2010 to analysis distributed field temperature in per pass in multi-pass butt joint welding. Using the heat source of GMAW model Goldak's double ellipsoid for the Auto-FCAW process. Prior to the results discussion, this paper also shows the step-bystep procedures to simulate which begins with metallurgical and customized heat source modeling, and is followed by creating geometrical mesh using Visual-Mesh 6.5 for analyzing and processing the results. Final objective of this research is aimed to be a baseline study to provide preliminary information in preparing the tools and equipment for experimental investigation.