Aiming at the overlaying of 690 nickel base alloy on S271 low alloy steel for nuclear power plant,the influence factors of single layer single pass welding dilution are studied.The results show that the dilution rate of weld is related to welding speed,wire feed rate and welding current. When the welding current and welding speed are constant,the dilution rate reduces as the wire feed rate increases;when the welding current and wire feed rate are constant,the dilution rate increases as the welding speed increases;when the welding speed and wire feed rate are constant,the dilution rate increases as the welding current increases. However,when the heat input,welding current and voltage are constant,changing the wire feed rate and welding speed will hardly affect the dilution rate of weld.