Abstract: The software of SYSWELD was used to build model and simulate the thermal cycle of in-service welding onto active pipeline of X70 steel using water as flowing medium.Influence of flow rate, pipe wall thickness and heat input on thermal cycle of coarse grain heat-affected zone was studied and the simulation results were tested by measuring thermal cycle on experiment pipelines.The results show that t_8/5 decreases when flow rate increases but the decrease amplitude is not so obvious.When flow rate is less than 0.5 m/s, t_8/3 and t_8/1 rapidly decrease while flow rate increases.When flow rate is greater than 0.5 m/s, t_8/3 and t_8/1 decrease slowly while the flow rate increases.t_8/5 and t_8/3 increase with the pipe wall thickness increasing and arrive at the maximum when wall thickness is 8 mm, and then decreases.But t_8/1 increases when wall thickness increases from 5 mm to 12 mm.t_8/5, t_8/3 and t_8/1 increase with heat input increasing.The simulation results of the thermal cycle agree well with the measured results and the relative error is less than 8%.