The field strength distribution of the magnetic circuit vs the NdFeB permanents was calculated in the condition of the same magnetic structure. The permanents were possessed of the same volumes and configurations, but different performaces, and there was the same width of air-gap in the magnetic circuit. Their field strength magnitudes were compared and the factors of influence on magnetic field strength were analyzed . According the results it is necessary comprehensively to consider magnetic-energy-product, coercive force, remanence etc rather than to emphasize the materials with high magnetic-energy-product upon designing the permanent magnetic field for magnetic refrigeration so as to provide the proper basis for selecting materials in designing the magnetic field for room-temperature magnetic refrigirators.