Abstract: In pebble-bed high temperature gas-cooled reactor (PB-HTGR), the fuel sphere's motion in the core is the basis of reactor physical design and structural design. The research of sphere's motion law plays a basic role in developing PB-HTGR technology. At present, there is no way to detect the sphere's route in real core and the corresponding theoretic and experimental research on sphere's motion law is still insufficient. To research sphere's motion law in core by experiment, an apparatus, which has the same shape and internal structure with the actual core of PB-HTGR but scaled down, was designed to simulate fuel sphere's motion in the core. To trace the route of target ball in the apparatus, a Cobalt-60 spiral CT system was developed. The verification experiment shows this CT system can fulfill the experimental apparatus's requirements of tracing target spheres' route.