The software of SYSWELD was used to build model and simulate thermal cycle of in-service welding onto active gas pipeline.Influence of pipe diameter,wall thickness and heat input on thermal cycle was studied.The results show that t8/5,t8/3 and peak temperature of inner surface decrease when wall thickness increases from 5 mm to 12 mm.But t8/1 will increases with the increase of wall thickness and will decrease after the wall thickness is larger than 7 mm.Pipe diameter has little influence on thermal cycle and that influence can be ignored when pipe diameter is greater than 273 mm.t8/5,t8/3,t8/1 and peak temperature of inner surface will increase with the increase of heat input. The software of SYSWELD was used to build model and simulate thermal cycle of in-service welding onto active gas pipeline. Inluence of pipe diameter, wall thickness and heat input on thermal cycle was studied. Results show that t8 / 5, t8 / 3 and peak temperature of inner surface decrease when wall thickness increases from 5 mm to 12 mm.But t8 / 1 will increases with the increase of wall thickness and will decrease after the wall thickness is larger than 7 mm. Pipe diameter has little influence on thermal cycle and that influence can be ignored when pipe diameter is greater than 273 mm.t8 / 5, t8 / 3, t8 / 1 and peak temperature of inner surface will increase with the increase of heat input.