The invention discloses a relative burn-up measuring system for a fuel tube. The relative burn-up measuring system comprises a three-dimensional transmission mechanism, wherein a mounting seat is arranged on the three-dimensional transmission mechanism and can move in a three-dimensional direction; a fixture is mounted in the mounting seat; a collimator is arranged outside the three-dimensional transmission mechanism and is connected with a gamma spectrometer; a collimating aperture of the collimator is aligned at the fixture; a center line of the collimating aperture of the collimator is perpendicular to a center line of the fixture. According to the system disclosed by the invention, a radial fuel tube burn-up measurement technology is established, and an important basis is provided for rapid judgment of male and female faces of the fuel tube; a technology for measuring the relative burn-up of the fuel tube is established, and a data support is provided for determining the burn-up of the fuel tube; moreover, the problems of relative burn-up measurement of the fuel tube and determination of the male and female faces of the fuel tube are solved, and a new thought is provided for relative burn-up measurement of different types of fuel elements.