The utility model discloses a magnetic powder detection test block assembly for incomplete welding of a root part of an iron magnetic thin-wall pipe with the wall thickness of no more than 4mm. The assembly is composed of three contrast test blocks, wherein the size of each of the three contrast test blocks is that the width is 150mm, the length is 400mm and the wall thickness is 4mm; a line cutting groove is formed in the middle position of each contrast test block and each of parts which are 50mm far away from the left and right sides of the middle position; the depth of the line cutting groove in the middle position is 0.6mm and the depths of the left and right line cutting grooves are 0.4mm and 1mm respectively; the lengths of the line cutting grooves are equal to the widths of the contrast test blocks; the widths of the line cutting grooves of the three contrast test blocks are 0.5mm, 1mm and 2mm respectively. The test block assembly is simple to manufacture and convenient to use, and can be used for judging whether the sensitivity of selected equipment meets the requirements or not so as to guarantee the reliability and the accuracy of a detection result.