Abstract: Experiments prove that the relations between the natural circulation capacity G and the heating power Q, and the total flow resistance Δp_f and the natural circulation capacity G are: G~Q^m and Δp_f~G^q, respectively. Power indexes of m and q are defined as the natural circulation flow rate-heating power characteristic number and the natural circulation flow-resistance characteristic number, respectively. In consideration of the effects of local resistance and frictional resistance, the analytical relations about thermal parameters are obtained by using approximation and fitting approach, and they also have good agreement with the experimental results. It is indicated that the natural circulation flow rate-heating power characteristic number m has a close relation with the flow resistance characteristics, and it is a function of Reynolds number Re and the ratio of the local resistance coefficient and the frictional resistance coefficient R_n in the natural circulation system. Its value decreases with the increasing of Rn. In addition, the relation between the natural circulation flow-resistance characteristic number q and the natural circulation flow rate-heating power characteristic number m is: q =（1-m）/m, which is validated by the experiment.