Abstract: The material of closed busbar is mainly aluminum or aluminum alloy, and its sound speed is greatly different to steel. The current standards provided that the detection of such welds required a special aluminum block, thus increasing the cost of testing. The testing method without aluminum block was discussed. The calculating formula and value range of K, and the drawing method of DAC curves were introduced. Detecting examples showed that the testing method was accurate in position with positioning error being less than 2%. In addition, to evaluate the defects on the busbar of diameter <159mm, an aluminum reference block should be used to produce the DAC curve.