Coiled tubing operation technology is widely used in drilling and repair operations, and the core of the technology is coiled tubing. The quality of the bias weld directly determines the fatigue life. Whether the microstructure of the bias weld is consistent with the base material determines whether the fatigue resistance of the weld reaches the level of the base material. This paper uses multifrequency eddy current testing technology to detect the reactance of the CT90 and CT100 coiled tubing steel strips in the bias weld area and the base material area to distinguish the microstructure and grain size of the base material area and the weld area, and judge whether the microstructure of the bias weld reaches the level of the base material. According to the signal characteristic cloud map, the weld area and the base material area are identified. The results show that the grain size of the coiled tubing bias weld area is significantly larger than that of the base material. It can be seen that the weld quality of CT90 is better than CT100. According to the microstructure difference between the weld area and the base material area, the quality of the bias is evaluated, which has guiding significance for the quality control and improvement of the coiled tubing production process.