Original position statistic distribution analysis (OPA) technique is a new kind of analysis method that can be used to detect the segregation, porosity and non-metal inclusion of iron and steel material. In the present paper, the technique has been applied to analyze two middle and low alloy welding plate samples. One sample is normal and there is no any defect in its welding seam, whereas the other sample has one obvious little defect in its welding seam. For each sample, both the welding seam and the mother material were scanned and analyzed, and the continuous content distribution of each element in scan area was thus accurately obtained. From the content distribution map, the weld shape and the transition zone were directly observed. By using the content curve from mother material to the welding seam, the content change range and its change gradient were calculated. These two parameters were suggested to evaluate the quality of the welding seam and the welding technology. For the sample with the little defect, in its defect region the contents of C and S presented higher, whereas the contents of metal elements such as Ni, Cr, Mn, Mo, Al presented lower. This result gave the reason why its mechanics property was relatively bad. It was suggested that the defect completely resulted from another kind of material.