Abstract: Three different modes for capturing heavy ions in CRing were investigated based on the study of longitudinal motion in HIAF. The typical beam of ²³⁸U³⁴⁺ was injected from BRing with energy of 800 MeV/u, and its longitudinal momentum spread was ±5×10^-4. The distribution of particles with different RF curves in longitudinal phase space was simulated using the code ESME, and the adiabatic capture mode was chosen as the basic working mode for CRing according to the simulation result.