Abstract: In CFR600, the precision of the calculation of control rod worth is of vital importance, due to that the control rods are the only way for reactivity control and reactor shut-down. In the CFR600 nuclear design calculation, the volume-based homogenization is adopted without any consideration of the heterogeneous effect, which actually is very significant for control rod and needs to be corrected. In this paper, the method for the group constant correction due to the heterogeneous effect was studied, and equations of two methods were derived, i.e., the flux-weighted method and the reactivity equivalence method. Based on the fine-mesh finite difference code, a calculation code named CREC for the group constant correction was developed. With the CREC code, the 12-group constants of the safety rods and shim rods used in the CFR600 were calculated. The calculation results of the heterogeneity correction of the control rod worth were obtained and preliminarily verified. The results obtained by the flux-weighted method and the reactivity equivalence method agree well with the reference value, indicating that these two methods are effective.