Abstract: To evaluate the welding quality of the dissimilar metals of D6A and 20 steels by robot automatic welding with X-ray test,the testing processes were arranged reasonably according to the welding structure,and high quality testing on the elliptic multil-layer welds and accurate characteration of flaws were achieved by elimating disturbance of weld appearance.The formation mechanism of cracks at the root of 20 steel with good weldability was analyzed.It can be attributed to great difference of thickness and tensile strength between the two kinds of steels,which leads to the cracks along bond line at the root of 20 steel by welding stress and tensile stress.According to the analyzed results,welding procedure was improved and cracks were restrained effectively.