By introducing a concept of shielding characteristics, a new method is proposed for shielding calculations of spent fuel shipping casks. The method separates ordinary shielding calculation into two steps, one calculates the radiation current leaking from the unshielded cavity region. The other method synthesizes the radiation dose rate outside the cask arising from the leaked current, the response functions for the radiation dose rate at the outer cask surface, and the functions for the radiation current reflected from the inner surface of the cask wall. In the synthesis, the effect of the coupling of the currents reflected between the cask wall and the cavity region is taken into account. The validity of the proposed method is confirmed by applying the method to an analysis of the measured data obtained for a CRIEPI cask. Response functions, the established characteristic functions for radiation shielding capabilities, are calculated for several typical actual casks. Calculated results are summarized for the convenience of applying the proposed method to actual cases. The merits of the present study are (a) the calculational code of the proposed method deals with only matrix calculations in short-step programming and is suitable for a microcomputer for which input data of characteristic functions are supplied from floppy disks, (b) with large and high-speed computers, one can evaluate radiation dose rates on the outer surface of a given cask in very short machine time and with good accuracy, (c) by application of the characteristic functions, one can extract information that will improve the design of the cask walls to provide more effective shielding by intercomparison of characteristic functions for several types of casks, and (d) one can foresee the influence of changes in the energy spectrum of source radiations on the neutron and the gamma-ray dose rates at the outer cask surface by the rule-of-thumb of superimposing the characteristic functions of the dose rate because they are functions of the incident energies.