ThesoftwareofSYSWELDwasusedtobuildmodelandsimulatethermalcycleofin-serviceweldingontoactivegaspipeline.Influenceofpipediameter,wallthicknessandheatinputonthermalcyclewasstudied.Theresultsshowthatt8/5,t8/3andpeaktemperatureofinnersurfacedecreasewhenwallthicknessincreasesfrom5mmto12mm.Butt8/1willincreaseswiththeincreaseofwallthicknessandwilldecreaseafterthewallthicknessislargerthan7mm.Pipediameterhaslittleinfluenceonthermalcycleandthatinfluencecanbeignoredwhenpipediameterisgreaterthan273mm.t8/5,t8/3,t8/1andpeaktemperatureofinnersurfacewillincreasewiththeincreaseofheatinput.