Abstract: Base on a CMOS coaxial visual sensing system,the optical radiation feature of the keyhole and weld pool in Nd:YAG laser full penetration welding was analyzed,and the detection of the weld dimension was realized.The results show that,when the welding was proceeding in keyhole opening mode,a low brightness field can be observed inside the high brightness of laser irradiation area.With the increasing of the welding speed,the dimension and the attenuation degree of the low brightness field are decreased,and moved to the hind weld pool.By the analyzing of the variation of the attenuation degree and the keyhole front wall angle,the relationship between the optical radiation feature and the weld dimension was established,and the detection of the weld dimension was realized.