The domestic manufactured CT80, Φ33×3.18 coiled tubing is supplied as heat-treated low-alloy steel. The relationship between thermo cycle, microstructure, tensile properties and hardness were researched for butt welded joint. The results show that CT80 is very sensitive to welding heat input, and the thermo accumulation was obvious for small dimension coiled tubing, varying degrees of softening always exist in HAZ. The reasons include growing of block ferrite, decreasing and isolating of continuous directional bainite. In order to get good properties, the strength of consumable welding wire should be properly matched with base metal, keep uniform strength distribution along welded joint, and avoid the plastic strain concentrate in local area.