Abstract: A relaxation test device for the hold-down leaf spring was designed considering the characteristics of the hold-down leaf spring for AFA3G fuel assembly. The reliability of the device is verified by a series of simulation tests. Finally, four groups of different recycled AFA3G fuel assemblies were selected, and the compression force and the deformation of the four compressed plate springs were accurately measured at any position in the whole stroke, using this relaxation test device. The results showed that the repetitive accuracy and the comprehensive accuracy of the device was 0.49% and 1.7%，satisfying the requirement of relaxation test.