Abstract: The penetration information can be extracted from weld pool geometrical size to some extent, so for realizing weld penetration automatic control, it is very important to obtain weld pool geometrical size accurately and reliably.For MAG arc welding, the deposited metal piles in the surface of workpiece and forms an angle between weld pool top surface and workpiece top surface.Thus an error of weld pool length detected based on visual sensor is arised.In the paper, factors which result in the error is analyzed and a method is put forward to revise the error.The gradient data of weld pool top surface are measured form side direction.According those data, the weld pool length is revised and more accurate weld pool length is obtained.