The utility model relates to a comparison test block for detecting the fusion depth of a U rib of a steel box girder. The comparison test block comprises a steel plate (1), wherein a probe (3) is arranged on one side of the steel plate (1); a skewed slot (2) is formed in the other side of the steel plate (1); an included angle between the skewed slot (2) and the bottom surface (1.1) of the steel plate (1) is consistent with an acute angle of the U rib of the steel box girder. According to the comparison test block for detecting the fusion depth of the U rib of the steel box girder, the difficulty that no comparison test block is available for detecting the fusion depth of a fillet seam of the U rib of the steel box girder is fundamentally solved; the comparison test block is made of a plate material with the same design as that of the steel box girder; the comparison test block is simple in design, easy to operate and convenient to use.